"How did we miss this bug?" and "How could have we discovered this recovery issue before it was on the critical path for a public outage?"
I am pretty sure they would have done that - just that they did not include in the post mortem.
The first was answered in the postmortem. The second is something done in time - either it's hard to answer in detail without revealing confidential information, or they are working towards it in the medium term.
I hasten to add that I'm not looking for finger-pointing or blame. In retrospectives, I think it's always best to assume that individuals did the best with what they had. [1] But I think it'd be great if Dropbox asked themselves things like "How did we miss this bug?" and "How could have we discovered this recovery issue before it was on the critical path for a public outage?" Questions like that help you solve not just this bug, but all the related latent bugs that you got the same way you got the one that just blew up.
[1] A lesson I learned from Norm Kerth: http://www.retrospectives.com/pages/retroPrimeDirective.html