microservices / clusters / zones - really all of these are other "cell-based" architectures as well. there is absolutely no written rule that a microservice was just an API or a singular service, it basically can be a independent instance that is testable/usable/gives value on itself.

that said: still a nice write up, learning about some of the architectural choices that AMEX makes is definitely insightful (and relavent/useful to what i am working on right now as well!)