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)