The group behind that project support the idea that people who use software should have access to the source code of that software, and therefore requesting the "source" is an action they would encourage.
They are also responsible for individual pieces of software with names like "GNU Emacs":
Hopefully this information proves interesting and useful to someone, and I haven't merely made a silly joke less funny by explaining it.
This should leapfrog you in that specific direction: https://en.wikipedia.org/wiki/History_of_free_and_open-sourc...
That doesn't really clarify "the joke" for me. I don't see how that's a joke.