A lot of people use "GNU/Linux" and "Linux" interchangeably to refer to the origins of the OS and not just the software that is being used. Pretty sure if you ask RMS he thinks that all distributions of Linux should be called "GNU/Linux".

Also, not sure what that has to do with them having a version of their software that runs on OS X. They could easily use libraries that aren't glibc for the Mac version.

RMS would say its weird that you think there is anything other than GNU/Linux...

http://www.gnu.org/gnu/linux-and-gnu.html

http://en.wikipedia.org/wiki/GNU/Linux_naming_controversy

Not quite.

http://www.gnu.org/gnu/gnu-linux-faq.html#linuxsyswithoutgnu

True, but then again I can not see it running on a kernel alone.

So maybe on a Linux Platform they are dependent on the GNU toolchain.