Why can't a race condition bug be seen as the single root cause? Yes, there were other factors that accelerated collapse, but those are inherent to DNS, which is outside the scope of a summary.
Importantly: the DNS problem was resolved (to degraded state) in 1hr15, and fully resolved in 2hr30. The Droplet Manager problem took much longer!
This is the point of complex failure analysis, and why that school of thought says "root causing" is counterproductive. There will always be other precipitating events!
† which itself could very well be a second-order effect of some even deeper and more latent issue that would be more useful to address!
The initial DynamoDB DNS outage was much worse. A bog-standard TOCTTOU for scheduled tasks that are assumed to be "instant". And the lack of controls that allowed one task to just blow up everything in one of the foundational services.
When I was at AWS some years ago, there were calls to limit the blast radius by using cell architecture to create vertical slices of the infrastructure for critical services. I guess that got completely sidelined.
1. How did it break?
2. Why did it collapse?
A1: Race condition
A2: What you said.
Nobody is saying that locks aren't interesting or important.
The race condition was necessary and sufficient for collapse. Absent corrective action it always leads to AWS going down. In the presence of corrective actions the severity of the failure would have been minor without other aggravating factors, but the race condition is always the cause of this failure.