It depends entirely on how AWS architected their power redundancy. Given that the outage affected a portion of one DC in one AZ, we can make some assumptions, but the truth is we just don't know.

It could be that their shared-fate scope is an entire data hall, or a set of rows, or even an entire building given that an AZ is made up of multiple datacenters. I don't know that AWS has ever published any kind of sub-AZ guarantees around reliability.

Datacenter power has all kinds of interesting failure modes. I've seen outages caused by a cat climbing into a substation, rats building a nest in a generator, fire-fighting in another part of the building causing flooding in the high-voltage switching room, etc.