The preface has "I initially wrote this book with talented high-school students in mind, particularly those with math-olympiad type backgrounds."

Apparently the author tried to somewhat expand the audience from that, but to me it seems still mostly appropriate for smart high schoolers who have heard some pieces of lore from friends about these topics, but they can't put that puzzle in order in their minds yet.

It's most definitely not aimed at the average student. You need to be highly curious, motivated and find math fun already.

And I think that's a perfectly fine thing. It's great to have books for that kind of audience.