Well, it's just based on my own observations. There's a relationship to Meyers-Brigg, but I don't think it corresponds exactly.

I'd bet PG is neither INTP or INTJ; he seems like ENTP. An introvert isn't going to start something like YC where you talk to hundreds of people, and manage hundreds of companies.

P vs J or perception vs. judgement doesn't quite characterize it either. I'm specifically talking about a way of approaching the work of programming. A big difference is that MBTI is supposed to apply to the entire population, where I'm just talking about programmers -- less than 1% of people. I think it's possible to describe/categorize the smaller group more accurately.