The way you lose portability by moving to a high level language is that you don't have an implementation of that high level language for as many platforms. You're not losing abstract portability in the sense that the semantics of the high level code is very highly machine-independent. But an actual port requires an actual toolchain.

C is not inherently portable, and it takes a lot of know how, care, testing, and sometimes luck, to write C that is portable.

C is de facto portable in the sense "able to be actually ported" by having the tools --- but possibly at the cost of combing the code for nonportabilities that have to be uprooted.