At this point UNIX == Linux for the overwhelming vast majority of users/systems out there. I'm really not a fan of this monoculture, but Linux is quite efficient and there are benefits to everyone using more or less the same platform.

POSIX isn't a great standard at all, but it doesn't really have to be. It's more or less been the "lowest common denominator" for a system for awhile, and it's "ok" at that.