Yeah, that’s a really odd take. Git is completely usable apart from GitHub and I would wager that the majority of Git usage happens without GitHub. I personally have far more personal repos hosted on a NAS than anything in GitHub, and while I did work for one company that used GitHub professionally, I can’t imagine it’s anywhere near ubiquitous. Prior to that I worked at a company with 10s of thousands of internal git repos and before their migration to git used a git front end for perforce. Prior to that (and prior to GitHub), I used a git front end for subversion.
I see GitHub as the successor to SourceForge, not a requirement for using git.
I guess we are just in very different professional circles. I've worked with dozens of teams over the last 10 years or so, and I've never come across anyone that self-hosts a repository. The vast majority use Github, and there's been a couple of Bitbuckets, and a couple of GitLabs. That's it.
Prior to that there were a few Subversions, a Perforce, MS's thing (TFS?).
You'll run into them if you ever work for organizations that have strict IP constraints or have no interest hosting their code on a platform in the US.
Coincidentally I have seen that becoming more and more common in my circles since last November.
I see GitHub as the successor to SourceForge, not a requirement for using git.