If you can't handle a single az failure there is no way you are going to handle failing over across different cloud providers correctly.
This is not true. Amazon is not being upfront about what happened here. It was simply not a single AZ failure. Our us-east-1 ELB load balancers were hosed and were unable to direct traffic to other AZs - they simply stopped working an were dropping traffic. We tried creating load balancers in different AZs and that didn't work either.
How can you be resilient to single AZ failures if load balancers stop working region wide during a single AZ outage?
Given the number of AWS global services that have dependencies on infra in US-EAST-1 (and, from the impacts of this and other past outages, seen vulnerable to single-AZ failures in US-EAST-1) that's...less avoidable for certain regions/AZs than one might naively expect. Most clouds seem to have at least some degree of this kind of vulnerability.
Not advocating for multi-cloud though...