Do you mean POSIX? In which case you should be aware that it actually costs money to get licensed as a POSIX-certified OS. If you actually meant UNIX, then of course GNU (GNU is not UNIX) isn't UNIX. It's in the name. At best the Linux kernel is a cousin of UNIX.

I meant Unix, but I should have said unlike all Linux distros except Inspur K-UX. I thought there were more but WP doesn't list any and seems comprehensive otherwise.

My point was really that some/most of those complaints about "Unix" don't apply to OS X for most people and sound more like the pain that users running Linux have to deal with, even though it's not typically an official Unix. I hope this was more clear.