Isn't that just MongoDB frozen in time before it was re-licensed under SSPL? Or is it actually a completely new query/storage engine that is MongoDB client-compatible?
In any case, I don't really like any of Amazon's homemade databases after the disaster that SimpleDB turned out to be.
DocumentDB an entirely separate implementation built on Amazon's internal tech which just speaks the MongoDB wire protocol. If they use any pre-SSPL MongoDB code it's just incidental bits and pieces.
Side note: From my experiences with documentdb, during the fires I had to put out trying to migrate a large mongodb workload to it, it looks like its Postgres or Aurora-Postgres under the hood (based solely on duck-typing around features, identifier constraints, storage limitations, billing, etc)
In any case, I don't really like any of Amazon's homemade databases after the disaster that SimpleDB turned out to be.