I think this has more to do with the phrasing of the post rather than our interpretations. Why would he add "my INTJ friend hates writing and isn't very good at it, so this leads me to believe it's just a Ne thing and not necessarily a intuition thing in general" if he was just saying that it was useful?
Sure, I may have misinterpreted what he meant, but I just interpreted what he wrote. Any well-developed function would be useful for something as complex as writing.
I would interpret that as him attempting to get across the point that Ne may be more correlated with writing, whereas perhaps Ni isn't. Again, this wouldn't mean that people with Ni cannot write well, but perhaps that Ne may be more naturally connected to linguistic ability, over Ni. Again, there's no way of really telling what the true difference is between Ni and Ne when it comes to linguistic ability, so there's no reason to really go into that area. All we can really say is that Ne does tend to have something to do with writing ability.
Although, I disagree with your last statement. Writing is a particular ability. Surely, if you're logical, your written 'content' will base itself more around logical structures and subjects. If it's introverted, it will be more analytical and subjective. If it's extraverted, it will be more objective and factual. If you have a highly developed Fe, your written content may focus around relationships and feelings. If it's introverted feeling, it will center around emotional understanding. And so on and so forth. But the point which is missed here is that functions will only really determine, or affect, the content of that which is written, whereas perhaps Ne (and maybe Ni) truly have some connection to "HOW one writes," rather than "WHAT one writes ABOUT."
Although, whether one is introverted or extraverted may also factor in here, as ENTPs seem to be rather introspective for an extraverted type, and introverts obviously tend to find writing more natural. Thus, introversion will probably draw a person to write more often, and enjoy it, whereas some kind of intuition may be more responsible for linguistic ability (and definitely, Ne seems to be very correlated to writing), although I'm not so sure about Ni, but I think it probably is correlated as well.
And the only sensing types I can see which are usually related in some way to writing are ISFJs and ISFPs (as these two types tend to be pretty artistic and creative). Although, there doesn't seem to be many famous authors who are considered ISFP or ISFJ. In general, famous authors tend to be intuitive types, having wild imaginations and a love of creative wordplay.
Again, INFPs are possible the greatest "writing" type.
INFPs have a natural aptitude for writing.
William Shakespeare if often considered one. And again, ENTPs, ENFP, and ENFJs all seem to have a knack for writing, and I've also noted that ENTJs tend to also have a way with words, using language with a sense of command.
ENTJs naturally write with an authoritative voice.
Among this group is obviously, INTP, INTJ, and INFJ. All of these types love to write, and do it without much effort, generally.
I would say that ENTPs, ENFPs, ENTJs, and ENFJs tend to enjoy the creative aspect of writing, but may lack the introvert's focus, instead gaining motivation from outside sources. In other words, by bouncing ideas off of others. Although, the ENTJ and ENFJ can easily structure and organize a paper, whereas ENTPs and ENFPs tend to write in fragments (which is perhaps why many ENTPs seem to love the art of rhyming).
Additionally, for most intuitive types, the following is true:
*Writes quickly, allowing one idea to suggest another.
*Make sure facts support imaginative ideas
For most sensing types, the following is true:
*Have difficulty deciding what to include in the paper.
*We encourage sensors to include more creative writing and examples if appropriate.
Thus, we can clearly see a pattern. Sensing types can be creative writers, but are often times more drawn to things like painting or sculpting (where they can work with actual objects, rather than immaterial ideas). Most intuitive types do not tend to have problems writing (although they have various strengths and weaknesses).
Ne, I would say, is definitely correlated to "easy writing," and the creative use of language, and the mastery of language, in general. Sensing types may be technical writers, or may write well if it involves something tangible. Ni is also obviously related to easy, creative writing, although I'm not entirely sure to what extent. Ne allows for brainstorming which allows one to imagine many possible words all at once, which in the INTPs case, must be strictly weeded for the best words possible, in a technical sense, whereas other intuitive types probably choose words in a more creative fashion—choosing a word because it's going to be more emotional or powerful, rather than because it's the most logical, accurate, or precise.
Although, I suppose it would be great to point, for sure, that it's not only Ne which is correlated with writing ability. Ni is as well, although many other functions don't seem to be so much. And again, cognitive abilities seem to suggest what is going to be written (subject-matter) rather than how one is going to actually transfer thoughts into words. I would look to the intuitive functions for this type of creative thought-to-word ability, mostly. Sensing seems more at home with thought-to-action, if anything.
Hence, there is no real relativism here, that I can see. Not all cognitive functions render people equally capable of writing skillfully, easily, and naturally (although it is true that skilled writers can come from any background). But if you look at patterns, intuitive types (specifically Ne) seems to have a lot to do with how well someone will use their words. Sensing seems more correlated with building, maintaining, operating, doing, and such. Intuition seems more correlated with ideas, thoughts, fantasy, and vision. Hence, it really isn't hard to see why intuitive types would naturally prefer writing to sensing types. It's no different than how Extraverted thinkers enjoy organizing the environment, whereas Introverted thinkers tend to enjoy organizing ideas and thoughts. Does it mean both types of thinking are equally great at organizing and managing people? No. Introverted thinkers don't tend to be great at organizing or managing people, whereas Extraverted thinkers seem to find it natural. Additionally, Introverted thinkers enjoy structuring and classifying ideas, whereas extraverted thinkers do not really prefer this.
So every type has inclinations for specific activity. This activity will be preferred over other activity. Hence, cognitive functions create APTITUDE in certain areas. For instance, I have a natural ability to think in the abstract and weed out logical inconsistencies (as many of you may as well). I do not have an aptitude for managing people, using my body creatively, or for nurturing and counseling people who are going through emotional distress. There are other people out there with preferred cognitive functions who can do what I cannot, and I can do what they cannot. Although, surely there are people with balanced functions, and they will find it reasonable to believe that they can everything somewhat decently. But most people aren't completely balanced, functionally. They have preferences and their activity is inclined toward certain areas and subjects.
So sure, the guy may have been a little off when he said that an INTJ may not find writing easier than an INTP, due to NE, because writing seems to be a natural things for all intuitives, and especially the introverted ones. Although, again, perhaps he did have a point by attempting to point how people with Ne tend to have a very particular way with words, which may perhaps be different from that of those intuitives with Ni. It's a possibility worth considering, so that there is no need to really get stuck on one particular phrase, simply because the OP may have implied something a bit untrue. Even if INTJs love writing, in general, as much as INTPs (since an inductive conclusion, that since one INTJ doesn't like writing, they all generally don't, is rather fallacious), it may be true that Ne does give people a particular way with words. And it's that part of the matter which is most interesting. That Ne is COMPLETELY responsible for writing ability is ridiculous.
Learn to distinguish when someone is wrong in some minor respect and when they are actually hitting on some large, general matter which is actually worth the harmless speculation. That is what few did in this thread.