What does Unix have to do with the Linux kernel? *nix is used for various "Unixes"
What's a "Unix"?
Is it a system based on AT&T code? If so, how much AT&T code has to still be in it.
Is it a system that passes the Single UNIX Specification test suite and whose supplier is thus allowed to license the "Unix" trademark?
Is it a system with a Unix-compatible API?
formerly also because of possible trademark issues. Linux is not one of them.
Linux is not one of the first types of OS in that list (if there are any bits of code AT&T made publicly available that are in Linux userlands, they're probably small enough not to count), and I know of no Linux distribution that's passed the SUS test suite (unless K-UX is a Linux distribution), so no Linux distribution I know of is one of the second types of OS in that list.
Linux is (or, rather, most Linux distribution are) most definitely one of the third types of OS, and people do speak of those OSes as "Un*xes", at least, even if "*nix" is used only for the first type of OS.