[k__]

"AWS DocumentDB seems to be pretty much tied to the MongoDB API"

I could imagine that they didn't build DocumentDB from the ground up.

DocumentDB is probably just a MongoDB compatible API for one of their base services (S3 or DynamoDB).

As far as I know, they build Serverless Aurora on top of S3, with the help of S3 select. So they will probably just create another custom-DB compatible API if they have the impression that this custom DB becomes the next big thing.

[jsteemann]

Exactly, AWS DocumentDB is only MongoDB API-compatible, but it's not using any MongoDB components.

It's an implementation of its own, leveraging many the base building blocks and infrastructure Amazon has created.

DocumentDB is currently tied to the MongoDB 3.6 API, that means all the transactional extensions MongoDB has added recently is not present in DocumentDB (yet).