'AND THEY WILL ALWAYS DO IT CHEAPER BECAUSE THEY DONT HAVE TO FUND FURTHER DEVELOPMENT.' Well, they DID actually contribute patches as the whole ES project was 'upstream-first', this is mentioned in the post with proofs.

From what I can find online, it looks like DocumentDB is actually a layer on top of the AWS Aurora PostgreSQL offering. This means they created a MongoDB compatible API, but are not offering a hosted mongodb cluster. (https://news.ycombinator.com/item?id=18870397)