And it was only one example, another one would be any kind of networking, as it isn't part of ISO C.
https://github.com/jart/cosmopolitan/tree/master/libc/sock
..and that also seems to have WinSock support:
https://github.com/jart/cosmopolitan/blob/master/libc/sock/c...
(also Windows is probably the only non-POSIX OS that matters - and APE is obviously only portable to systems it specifically supports, it can't support systems it doesn't even know about, no matter if it is a "POSIX system" or not).
And it was only one example, another one would be any kind of networking, as it isn't part of ISO C.