Yes, if the question were "How many people go to a doctor to complain of symptoms of mental illnesses" then sure, a large chunk of the general American population (still don't know if a majority) would qualify. However recall the context. We started with the question "Have you ever self-diagnosed yourself with a mental disorder?" and are talking about the follow-up to it. Here the question about going to the doctor means mostly "Did you take your self-diagnosis seriously enough to talk to a medic about it?" And, still within this context, the question is much more like "I think I'm mentally ill, is that so?" than "I can't sleep and life is pointless, how do I fix that?"