It's simple, really.
Git, the protocol is distributed. You can have a copy, I can have a copy; anyone can have a copy. And they're all equal.
Github is Git with a bunch of stuff that's not copyable. If you want to move your data elsewhere, you lose access to a host of stuff. Wiki, issues, PRs, branches, fixes.. All sorts of things.
There's people who've developed bandaids to triage that data lockin, but they only stem the bleeding. They don't solve the problem, and that problem is your data is in a roach motel.
What RMS keeps going on about much of the time is a high level engineer-y view of the problem. He forgets to bring it back down to earth, to explain that "Yes, these things I talk about affect us all!"
For me, I'm setting up a new server right now, and I'm putting Fossil SCM on it for my repo instead. It does handle all those data assets that makes github useful. And its license compatible with Free Software.
https://www.fossil-scm.org/index.html/doc/trunk/www/index.wi...
edit: I'm also a card carrying member of the FSF. I've lived through enough computer "revolutions" to identify bad stuff when I see it. I'd rather my knowledge be portable, with usage and configuration of free software, than lock it up in proprietary blobs that change underneath me.