I believe he's referring to the fact that many if not most projects hosted on github have no license conditions at all. No license means no one except the original author can use the software, and anyone who even looks at the code runs the risk of being sued for copyright infringement.

RMS is not saying "make everything GPL" (or at least, not in this context). He's just saying if you're going to publish source code, please make clear the condition under which it can be read or used.