To be fair: emacs was written in an era where the idea of a "standard" C library was kind of a joke. Those functions provide "no value" today on systems that have been POSIX-valid for decades. They certainly did in the mid-80's when Unix portability was the biggest hassle of the day.

That said, though: wouldn't that argue for things like "remove custom mkstemp" and not "rewrite it all in rust?"