The reasonable conclusion is that a bug exists where -- under certain circumstances -- rename(2) fails in a way that is not expected. It's a bug. It's not as if rename(2) is intentionally implemented in a way that is not atomic; it's that there is a bug. Should it be fixed? Yes. Does it mean that OS X is not Unix? No.
One may feel comfortable claiming that OS X is "Unix" based on the fact that all versions of OS X since 10.5 have been "Open Brand UNIX 03 Registered Product" (from an archive of the Apple website), signifying that they have met the requirements for the SUSv3 and POSIX 1003.1 specification.
