I would clarify and say that it's aimed at high school students with a serious thirst for competition math and math in general, Evan hangs around a lot (or at least he used to) on the Art of Problem Solving forums, and I think it's these students who spend their day pouring over math discussions, and math problems, that the book is aimed at.

These students would already know that Z is the set of integers, and if they didn't I don't think it would be a deterrent to pouring over the book.