This is not exactly true. The az names are indeed randomized per account, and this is the identifier that you see everywhere in the APIs. The difference now is that they also expose a mapping from AZ name (randomized) to AZ id (not randomized), so that you can know that AZ A in one account is actually in the same datacenter as AZ B in a different account. This becomes quite relevant when you have systems spread across accounts but want the communication to stay zonal.
You're both partially right. Some regions have random mapping for AZs; all regions since 2012 have static AZ mapping. https://docs.aws.amazon.com/global-infrastructure/latest/reg...
Oh wow. Thanks for telling me this. I didn't know that this was different for different regions. I just checked some of my accounts, and indeed the mapping is stable between accounts for for example Frankfurt, but not Sydney.