You want basic undergraduate probability and linear algebra and some calculus on the side, but you should get along with those. Also some practice with reading academic texts so that you can try to extract some useful meaning from it without understanding every part helps. Also you need some general familiarity with how academic math papers are written, the concepts in 2.1 aren’t complex (high-dimensional space make random points stick together in clumps less), but the way the book writes it is going to be unfamiliar if you haven’t been exposed to academic math writing much before.

Not sure what’s a good place to get that other than “go to university, minor in math”. Khan Academy?

It really needs a personal computer to schedule the repetitions, and we’re only now getting to the point where every schoolchild having their own handheld computer is a somewhat practical proposition.