Earlier quoted context omitted.
I'm not sure that it's that mind boggling. The entire complexity and value of GHA is not in the runners but the scheduler that the GH frontend hooks into and runners subscribe to jobs from. It's the most likely thing to fall over. ' If the scheduling was self hosted it would be inexcusable but you can always just connect whatever you want to webhooks.
That "scheduling" is just a git hook and a message queue, maybe with some database updates in between with very clear boundaries that make sharding easy to reason about, assuming they have a sane architecture (they evidently don't)
I don't disagree that it's obvious they've got problems but I'm just saying it's obvious to me the part that falls over (the scheduling of jobs) and why that would impact self hosted runners, which do no scheduling but depend on it to function.
As for 'just a message queue with some database updates and sharding that's easy to reason about'... Here's a job scheduling problem as an example: imagine you schedule a job, and there's no runner available. How do you disambiguate between no runners available because you've reached capacity, runners not being available because they're on a real network with faulty connections, and runners not being available because of a faulty rollout of internal updates?
A simple message queue for job scheduling is fine if you own everything and can deal with the operational overhead of identifying those cases by hand, but Github can't do that.