To me, this is a perfectly valid write-up with a good lessons learned. They have written it in a very diplomatic way, but to me, it is absolutely clear that Google screwed up here. How can you make such a change to a default behavior of critical infrastructure unannounced? That's just reckless towards your customers, and solidifies my belief to stay away from GCP.
If they had properly announced the change, even if the Firefox team hadn't then tested beforehand, at least the DevOps team would have put one and one together and just changed back to HTTP/2 and the outage would have lasted maybe 10 minutes. Instead, they frantically went through their git log to see what in the code base might have triggered this bug. Everyone who has been in such a position knows how incredibly stressful this is. I'd be absolutely livid at Google in their position. That it took two hours to fix this is clearly their fault.