I have ADD-I and thinking it doesn't exist is pure ignorance. please go read the many legitimate scientific studies out there before you judge something you really know nothing about.
I didn't know I was smart until I started taking meds, always thought I was just a dumb kid. I was diagnosed at 20 b/c at 15 the psychologist said I was too smart to have it. It's just hard to pay attention when you constantly are in a mental fog. All my life I had faked conversations and school lectures. The teacher asks at the end "anyone have any questions?" -- I'm nodding like I understand everything but in my mind it was yeah, repeat everything you just said, I didn't process that at all. I never followed conversations well unless they were terribly interesting, even then I always missed details and my mind would go into 10 second daydreams before I'd get pissed at myself and snap out of it. After the convo would end, they'd look at me to respond and I'd say ohhh yeaaah, *mutter some stupid statement agreeing with them* because I really only 1/2way followed what they said. You get sick of people thinking you're stupid and giving you that look of confusion when you ask them to repeat things so many times it's better just to stay quiet and nod and pretend you know what they're talking about. It's humiliating because you truly want to listen and understand, but what are you gonna do about it if you don't know it's ADHD?
When you have ADHD and take stimulants you talk to those same people, follow the entire conversation, and impress them with your INTP brain by pulling off connections left and right, throwing in insights, actually CONTRIBUTING like any smart person should be able to do. People start to think you're really smart, you become more confident, and you no longer write notes verbatim in class because you're actually LISTENING and wow, learning. You have motivation to do things, talk to people, READ. I never read until I got on meds because I would read the same sentence over and over. Now I am always checking out books at the library. It's painful to know how many years of my life I lost because of not being medicated, how many years I could have payed attention in school and not been humiliated b/c of my incompetence. If an INTP gains their happiness from intellectual ability and competence then having ADHD really is a terrible, terrible evil.