I think that AWS DocumentDB uses the last mongodb version that wasn't their new(ish) SSPL license and any compatibility beyond that was "clean room" backported. Its possible Azure is in a similar place.
DocumentDB is a thin layer that converts the mongo commands to Postgres, uses Aurora Postgres. If you check, the constraints are exactly same. Even list RDS CLI returns the DocumentDB clusters.
Most obvious indicator is DocumentDB is single node write whereas Mongo is really not, coming from Aurora Postgres.
