GNU/Linux was initially developed from scratch to behave like UNIX to a certain extent (hence the name "Unix-like"). In other words, it's a rewrite inspired from a model.
macOS has a marketshare in server market segment of around 0% (client of around 5%, iOS substantial though), and Linux is the defacto standard in that area. Which OS do people learn nowadays? Linux distributions. So it makes sense to see it from that PoV. Because chances are that you're going to use that defacto standard. And even if you prefer UNIX(tm) or BSD or Solaris you'd rather use Linux than Windows or OpenVMS. Cause the latter are aliens from outer space.
More philosophically, insisting on a definition of UNIX which includes z/OS and excludes Seventh Edition Unix isn't going to win you many friends.
http://www.opengroup.org/csq/repository/norationale=1&norefe...
And speaking of aliens from outer space, z/OS is a JCL-eating Martian speaking EBCDIC through its virtual cardpunch. ;)