>And here's a different, more controversial point. We used to have psychiatric disorders called hysteria and homosexuality. Now those have been reclassified out of existence.
I don't think that homosexuality is a good example to illustrate your point that ADHD might not be a "real" disorder. There's no question that there are people who are homosexual, but we no longer refer to homosexuality as a disorder because we no longer believe that it requires treatment. If ADHD were to parallel homosexuality in this regard, then there would be people who actually have ADHD, but we would stop viewing this as an abnormal variation in behavior.
Your example of hysteria is a better one to illustrate your point though. We no longer believe that hysteria is a disorder because we no longer recognize it as an actual state of being.
