BSD networking was distributed under a very permissive license, so that would extend to any new APIs included in BSD. You'd have to argue that the BSD license doesn't cover API rights, which would basically be like trying to argue that it's not really permissively licensed at all.
You'd probably have a better argument for GPL violation for non-GPL reimplementations, but I'm not aware of that many people doing proprietary reimplementations of Linux APIs. Hell, even Microsoft stopped trying that when they moved to WSL2.
