A lot can be said about AWS and open source, but it is clear they have created some "secret sauce" on the networking, storage and virtualization side of things which is their core business. Unlike Elastic they never promised open source and never used it as marketing. So it completely fair for them to keep those things closed as they provide a big part of the competitive advantage to their cloud.

Those components underpin things like Aurora (which does tricks with storage and replication that MySQL can't) and this warm/hot storage. So there is probably no practical way to open source those elasticsearch changes without opening up their storage system as well and even then it wouldn't run outside of AWS.