By what metric was his decision wrong?

If he's trying to maximize expected total wages over his career, staying in academia isn't a good way to do that. Although he'd probably be better off at a larger, more established company than at a startup.

If he's trying to maximize his career satisfaction, and he wasn't happy in academia but was excited about startups, he made a good decision. And I think that was the case here.

Some other confounding factors about his situation at the time:

• He'd just been accepted to YCombinator, which is a guarantee of mentoring and venture capital

• Since he already had funding, it's not like he was dumping his life savings into a startup expecting a return

• He has an open invitation to come back to his PHD program whenever he wants

If you still really want to blame someone for his decision, I think Paul Graham had a much bigger impact on him than anyone associated with LessWrong did.