What does that answer of his even mean? That he prefers mysql ?

I interpret it as "I didn't think about it, picked the first thing that worked in the past, and don't wish to make an argument about it, so let it go".

I think he means that if you use Postgres you are more likely to also be an evangelist for it. That doesn't tend to happen on the Mysql side. You don't hear people saying "you should switch from postgres to mysql" but you hear plenty of the opposite. Mysql works fine for most things and so that's what he used.

I assume it means his bottleneck has always been somewhere higher up in the stack :)

Now that I've thought about it more, it makes more sense. When used properly, Rails handles a bunch of the stuff that would make Postgres a better choice (like data validation, constraints, etc.)