This I disagree with. There are literally millions of people who suffer from mental health disorders that are never diagnosed, either because they avoid treatment, don't know how to seek help, or don't even realize that anything is wrong.
My grandmother on my father's side was one of the more mentally ill people I've ever encountered--the had the bipolar disorder endemic to the DeMillo family, she had panic disorder, and on and on--but she was never diagnosed by a medical professional or treated. She self-medicated and refused to acknowledge anything was wrong.
It's so, so, so common for mental illnesses to be undiagnosed.