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That sounds like a circular statement to me. But even if true, it's not ok to diagnose people on HN or to use diagnostic labels to juice up arguments.

Note how the GP's substance doesn't change if you take out the label "mentally ill". It just gets less labelly.

I don't believe that's what the post is saying though. It appears to be saying a minority of them are mentally ill.

Ad hominem arguments are always unacceptable. People with various mental illnesses (depression, autism, asperger, OCD, etc.) can still hold perfectly valid claims. The fact that somebody might see the world in a different way does not discredit them.