In the report they say they're looking at moving to a new region, but Google apparently told them that us-central1-a was down. The "-a" makes it an AZ. It sounds like they're only on one AZ and may not fully understand the difference.
[correction: they accidentally called usc1-a a region, but everything mentioned in their outage was a zone. They specifically called it a "deployment zone" not an availability zone, so it sounds like an issue of inexperience with best practices.]
[obligatory disclaimer: I'm a Google employee. I don't have a relationship with Layer]