Git was already quite popular before GitHub at least in technical circles. I was using it as my first version control tool when I was learning programming. I asked some programmer friends and the answer was: "use Git, it's 10x better than SVN".
Git is designed in a way that everyone has their version of the code locally. We were hosting our projects on another service back then. GitHub is the result of Git getting traction, not something that made Git popular.
Yeah, I was maybe exaggerating when I said "kernel devs". There obviously were people using git before Github.
But I don't think source control in general was as common back then as it is now. Source control was something for serious projects, and a lot of code wasn't under version control at all. If you wanted to host code somewhere, services typically charged per repository, and setting up repo hosting yourself was a hassle.
Github really made all that very easy, and popular. The alternative web interfaces for browsing git repos were atrocious, but even modern competitors like SourceHut are a hard sell to novices.
I think calling Github "just another Corporation build around Git" is pretty disingenuous.
The way Github presents repos organized by organisation, allowing everyone to fork and host their version of the code, that's what made git popular.
Without Github, I don't think Git would ever have been adopted by mainstream dev audiences and would just be some arcane tech used by kernel devs.