The beginnings of his exposure as being batshit crazy were there in Dilbert if you went back after he exposed himself. But the undeniable evidence that he's batshit crazy comes from his early books, long before he fell down into his "I'm a hypnotist so I know everything about persuasion" rabbit hole. He genuinely believed in affirmations as a hack code for the universe where writing down something and wishing for it hard enough will make the entire universe change to get it to you. His novels were so bad that he accomplished the impossible: he made me wish Ayn Rand had given him some writing tips. This is years before he fell down into the right wing grifter space he now occupies.
He was fucking crazy from his YOUTH. (Source: his own stories on his blog!)
People, by default, assume because he made stuff people liked that he was a sane, rational person. No matter how much talent or money someone has they're still as prone to having batshit insane takes as anyone else you may encounter.
This is a common problem. "I like this singer's music ... wait ... he said WHAT about black people!? But ... dude is BLACK!!!" "I like this painter's portraits. Wait, what did he do with children!?"