If all of glibc had been written in assembly for every single target triplet, one wouldn't be wrong to point out that there was no benefit to doing that instead of writing it in C, and that it probably took more work and was more error-prone, even if they weren't willing to help port said code to C.

Just the same, they could have written this in C++.

Much of LLVM's libc is written in C++ with exposed C bindings (or all, I haven't checked 100%). For example, their libc stdio implementation is completely in C++.

https://github.com/llvm/llvm-project/tree/main/libc/src/stdi...

You could say the same thing about Rust and Relibc, but it seems much less likely that a C project would incorporate Rust than that it would incorporate C++.