It was a DNS issue with DynamoDB, the load balancer issue was a knock-on effect after the DNS issue was resolved. But the problem is it was a ~15 hour outage, and a big reason behind that was the fact that the load in that region is massive. Signal could very well have had their infrastructure in more than one availability zone but since the outage affected the entire region they are screwed.

You’re right that this can be somewhat mitigated by having infrastructure in multiple regions, but if they don’t, the reason is cost. Multi-region redundancy costs an arm and a leg. You can accomplish that same redundancy via Colo DCs for a fraction of the cost, and when you do fix the root issue, you won’t then have your load balancers fail on you because in addition to your own systems you have half the internet all trying to pass its backlog of traffic at once.