He's saying he didn't realise that he had personal preferences (for grad school, about being trans) that made "objective" data incorrect, but because he'd learned from "rationalism" that most people that try and use their own special circumstances to override data end up with worse outcomes he decided to ignore the fact that he was miserable and everyone was telling him he was cis when he wasn't.

Unfortunately, he got out of the issue by saying he was a special snowflake who was above the masses, and if he wasn't actually that and was fooling himself the masses would crush him because obviously society is a malthusian hell.

Just...epic level brainworms. Basically what happens when you read "Thinking Fast and Slow" and use it as a bible for every single action you take.