I think a lot of people wrongly assume that GitHub is so "easy" that you don't actually have to spend any time to learn how to use it.
But that's not the case at all.
Moreover, GitHub has so many quirks, that most Enterprise shops don't use it internally for their own internal projects (unless they release said projects into OSS); so, when you work for any company that's not just a startup, you likely have to learn yet another set of tools. And again if you switch the company.
Compare this with email. As the original article reveals, Linux and OpenBSD happen to have almost entirely identical workflows for patch submission, which is actually REALLY weird, considering that the two projects have entirely different histories, methodology, and use entirely different revision control systems, where OpenBSD still uses CVS, yet Linus Torvalds hated CVS so much, I recall that he refused to use it even before Git was ready and the licences for the predecessor system have already been withdrawn (I recall he hated CVS so much he simply used plain patches or tarballs).