It's like saying the Taurus, Camaro and Camry are all varieties of Mustang. The statement is false. Those are all varieties of cars, or automobiles - but not varieties of Mustang. Similarly, the listed Operating Systems are not varieties of Linux...

Seems obvious he meant they are varieties of Unix.

Maybe. I've met an awful lot of people to whom such a distinction is just not obvious. People who would, for example, assert with perfect sincerity that MacOS is built on Linux, or that Solaris (or OpenSolaris, or Illumos, etc...) are the same thing as Linux. It may have been a typo, or it may not have been. The person I was directly responding to didn't seem to think so.

Anyway, that's where the sentiment is coming from.