True, but it's anything but the end of the world if GitHub vanished from the face of the Earth right now. Everything would be running full steam within a week.
There is team and org level authZ and authN data at github that is needed by companies working on cross-company projects. That data is not stored in code repos.
I disagree. Many OSS projects rely upon GitHub, especially projects that package or distribute other projects. Sure, everyone would eventually work out their own hosting, but it'd be a nightmare of changing URLs and uneven servers in the interim. I think a week would be extraordinarily optimistic.