> The build backend is an explicit part of the specification of a Python project

Nothing in the Python spec forces Gentoo to invalidate the downstream package every time build-system dependencies update - that's a behaviour they've decided is correct on their own.

> C/C++ libraries don't have an explicit specification of the build tool to be used in their project

They don't have a standard way to specify such, but they have an implicit dependency on the installed compiler toolchain, and it would be just as valid to interpret "no way to specify a build-system dependency" as "always rebuild" when the build tool updates