I was more curious on why they thought that would increase availability. It didn't seem like it would be constructive to speculate about why they chose that.

AWS doesn't guarantee that failures within an AZ are independent of each other, so it's not clear how you would estimate what availability you'd gain with this. Losing everything in an AZ + 1 instance sounds like a very unusual and specific scenario to design for.

The use case here is an AZ going down (reasonable to guard against) and an individual machine failure (the first reason we use HA like this anyway).

An AZ going down doesn’t make all other hardware reliable, and equally a machine going down from a cluster doesn’t mean that all AZs are going to be reliable.

Many products have uptime requirements above what Amazon can provide at the AZ or machine level.