The preprocessor does not add portability. It adds a path for you, the programmer, to succinctly specify additions and deletions from your program so as to make it portable.

This is markedly different from a language that, for example, guarantees the existence of integers of unbounded size. In this case, when working with integers, we make no mention of the platform to execute on. We are provided with an abstract guarantee that one can rely on regardless of the concrete platform.