One of the reasons Amazon was successful as a cloud hosting company, and offers all the services it does is because of the famous Bezos email/memo to the entire company saying every API has to be designed as if it were external facing (usable by 3rd parties).

I think what you are conflating is "good documentation" and "well designed APIs" as an attribute of REST, when it's really just an attribute of good engineering.

Engineers have used functional interfaces for decades and there's just nothing at all in REST that's any kind of improvement over functional. There's a reason "functional programming" is a thing and "nounical programming" isn't even a word.