> Even if this were true (which it isn't)

Maybe I was being too general, but the diagnosis criteria I remember from when I've looked at such (including ADHD, Autism spectra disorders, and Depression) have included such qualifiers, IIRC.

Checking ASD, the following is one of the criteria:

    The deficits result in functional limitations in effective communication, 
    social participation, social relationships, academic achievement, 
    or occupational performance, individually or in combination.

While the word "negatively affected" is not specifically used, I would at least argue that the above is morally the same.

Of course the diagnosing psychiatrist could always fudge the facts, but arguing whether or not psychiatrists accurately apply diagnosis criteria is a different matter to arguing that "to avoid one or another mental illness diagnosis, [...] you can't be extraordinary in any way".