On the basis of claims of high IQs: no. I could claim to have an IQ of 200, which I certainly do not have. A lot of people claim a lot of things that are often extravagant and untrue. It would be far too lacking in credibility to base a conclusion on claims only.
However, I do think that Internet forums, and the Internet in itself, tends to attract individuals that are more intelligent than the average. Moreover, some forums probably have higher average IQs than others, depending on the orientation of the forum. Excuse me for this as it is based on stereotypes, but I daresay the average IQ of a science oriented forum is higher than the average IQ is for a forum that is oriented towards monster truck enthusiasts.
Forums are platforms for socialization, debate, learning, self-identification, search of self, etc., in general things that are intellectually stimulating, that a person often will not get in real life. More intelligent people are more likely to seek such intellectual stimulation than less intelligent people. Meaning that the forums simply attract smarter people.
Furthermore, on the Internet you can find like-minded persons by simply using a search-engine; I'm confident that many here have done that, this being an INTP oriented forum. People cannot do that in real life, and therefore they often retreat to the Internet to find whatever it is that they seek.
Compare Internet forums to higher education institutions like college. The people that are attracted to colleges are in a way quite similar to the people on Internet forums. They come there for intellectual stimulation and to go deeper into their interests, maybe even find like-minded people if possible. The differences pertain to semantics.
Whether IQ is a valid measurement for intelligence is another topic. It is certainly not the sole factor we should rely on as intelligence is too abstract to be measured. Although it does give an indication. Therefore it should be noted that I do not necessarily define intelligence as being the same thing as having a high IQ.
So from a speculative and untested viewpoint: yes, I think that the average IQ on Internet forums are higher than in real life and I believe that they are more intelligent.