You still have your local source code, but GitHub gives you issue tracking, comments, pull requests, and other things you could still use for work - none of which can be cloned locally.
PITB, yes. But you can still get work done.
Plus you are forgetting that lot of automated jobs get triggered on github changes. Many shops kick off all kinds of tests, deployments, and other things based on changes to the github repo.
My point was simply that you can still be productive when github goes down, if you want to be.
I don't think you're "wrong" for the developer use case, but the reality is that it can bring large teams to a screeching halt.