I agree. Obviously there will be people who try to swing the lead in the system, but there is no evidence that they are very many. But the rest of us often talk without expert knowledge, yet talk in a way that does shine through with a pattern of signs and symptoms. Psychiatrists may have all kinds of criticism, but they can assess fairly accurately patterns of thoughts and emotions etc.
Taken on its own, self assessment has all kinds of problems, as can be seen i n people who do an internet quetionnaire and decide they have ten diagnoses. But it is not such a problem if in front of someone who has seen all kinds behaviours. We can argue over what they do with those patterns, but self assessment is needed.