I don't think anyone is overlooking the reasons why one might have more downtime than the other.
The problem is that ultimately the downtime itself matters and not the reason, and if you don't need any of the features that GitHub offers, then the self-hosted route is a better option.
With all due respect to the developers, it would not speak well for GitHub if they can't maintain stability while refactoring. There's a lot of testing processes and CI processes explicitly around making those actions safe.
New features breaking is a lot more understandable - even expected - than regressions and refactoring failures.
The problem is that ultimately the downtime itself matters and not the reason, and if you don't need any of the features that GitHub offers, then the self-hosted route is a better option.