The current infrastructure cannot handle the new load requirements. Either the infrastructure must change, or they must start denying users the ability to use the infrastructure.
I may have come out with a different interpretation than you did of GP's comment. I see how you got to yours. But the way I read them saying they should slow down was, maybe slow down on new features. Which would mean they could shift resources from new features to infra.
I think he meant to say "prioritize" so that availability related work sees results sooner — "acceleration" — than it would without an increase in priority.
It feels like GitHub maybe needs to slow down? 'We must change things faster' is a wild way to start off an eight hour hard-down postmortem.