The follow on questions on that thread seem to be fairly hypothetical handwringing, and something I’d expect the FSF to submit an amicus brief, giving their fairly reasonable FAQ answers, if it ever did show up in court. Given that the GPLv3 (and its derivative, the AGPLv3) allow for up to thirty days to remedy shortcomings, your source code being unavailable for a few hours while GitHub is down seems like a non issue.
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=495721#17
The follow on questions on that thread seem to be fairly hypothetical handwringing, and something I’d expect the FSF to submit an amicus brief, giving their fairly reasonable FAQ answers, if it ever did show up in court. Given that the GPLv3 (and its derivative, the AGPLv3) allow for up to thirty days to remedy shortcomings, your source code being unavailable for a few hours while GitHub is down seems like a non issue.