Their siloing strategy, which I'll roughly refer as resolving a request from a single AZ, is a good way to keep operations and monitoring simple.

A past team of mine managed services in a similar fashion. We had a couple (usually 2-4) single AZ clusters with a thin (Envoy) layer to balance traffic between clusters.

We could detect incidents in a single cluster by comparing metrics across clusters. Mitigation was easy, we could drain a cluster in under a minute, redirecting traffic to the other ones. Most traffic was intra AZ, so it was fast and there was no cross-AZ networking fees.

The downside is that most services were running in several clusters, so there was redundancy in compute, caches, etc.

When we talked to people outside the company, e.g. solution architects from our cloud provider, they would be surprised at our architecture and immediately suggest multi-region clusters. I would joke that our single AZ clusters were a feature, not a bug.

Nice to see other folks having success with a similar architecture!