"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.