I'm going to guess it's because Ubuntu was called "unix" (which actually raised my eyebrow as well). Although similar, unix != linux, and Ubuntu especially != unix, as opposed to a distro like Slackware, which is more or less unix with a linux kernel slapped in.

I got what the author meant though, and it's not that big a deal.

There is "Unix", an operating system originated at AT&T and "unix", a generic name that usually refers to a family of operating systems that are based more or less on the same ideas. Linux is a lot closer to AT&T's ideas of Unix than other certified Unixes like OSX and AIX. Linux is not Unix, but it certainly is a unix.

When I really want to annoy my BSD friends, I call it a "Linux-like" operating system. It never fails.