Earlier quoted context omitted.
>and so it was a long time after that before I was exposed to any math that was really more advanced than basic calculus. This suggests calculus was all he knew before at least college (for some value of "a long time").
I wound not be surprised if he is the product of a Math Circle:a nice place to stretch one's mathematical abilities while retaining the bragging rights of never having formally studied math in college.
I wouldn't either, but there's really no evidence to suggest that.