Because in fact all the changes were about changing the function signatures. The function signature is enforced by the linker. The semantic of strings that go through isn't.

Returning 2 values is possible in C (via pointers input parameters). The main issue here is that libc is a strong API contract based on C function signatures, that nobody can change, only augment with new symbols. Such strong API compatibility constraints happen in any programming language, and workaround to fit new behaviours in old APIs happens everywhere.