Well, everything has some amount of implied/customary prerequisites. You wouldn't complain if a books "Introduction to Metaprogramming in Ruby" or "Introduction to C++ templates" didn't teach you programming.

In math many "general topics" like calculus, linear algebra, probability etc are sometimes studied a few times (eg the second course in calculus might start from scratch and carefully construct the real numbers, which the first course skipped), and I've noticed some authors use "Introduction to X" to mean "this textbook is meant to be a first course in X"