Nor does it disqualify him.
Actually that's a really stupid assertion the more I think about it. based on your reasoning doctors should not attempt to diagnose patients by asking them what they ate, where they've been, what their symptoms have been or ask them, "What do YOU think caused this condition?"
More often than not patient testimony is the most accurate path to diagnosis and treatment of a condition. And in cases where the doctors have disqualifed themselves from scientific objectivity, as the mental health profession has done in this case, then patient testimony may be all we can rely on.