Interesting that the Under Attack Mode wasn’t used at all here. I understand not wanting to break APIs but I feel temporarily challenging non-API usage could have at least helped without impacting users too much?

I talked about that directly in the post. We didn't want to just challenge everyone. We use JS challenges but we try to use them sparingly. The rest of the ops team and I were fighting to stay up but it never got so bad that it was a choice between complete outage and using the Under Attack mode.