Hi, I work at AWS on Compute services, but not directly on database services. I also work on broader open source topics at Amazon.
Amazon DocumentDB was in development for a long time, and development started well before any of the license changes that MongoDB (the company) made to MongoDB (the database).
I believe in response to some licensing changes MongoDB made, so the AWS service targets the last permitted license release by Mongo.
https://aws.amazon.com/blogs/aws/new-amazon-documentdb-with-...
*I forgot to mention, they did not release the code for this as you may have been looking for apple-to-apple comparison with the ES situation.