This is FUD. It was FUD when this argument was introduced by a scummy marketing company with product they call a database, and it remains FUD in this case also.
1. AWS never provided a hosted Mongo instance. You know who did? Small startups that provided better support than MongoDB Inc., without the scummy "just dump money into MongoDB Atlas! It will solve all your problems! I promise!" spiel that the marketing company in disguise shoved down its customers' throats (even paying customers).
MongoDB Inc. went around buying out these startups for cents on the dollar right before they publicly announced the license change.
2. AWS did provide a hosted ElasticSearch service. And they did contribute code in return. Amazon's patches to ES included oft-requested security features that ES upstream flat out refused to merge because that'd harm ES corporate's monetization strategy of demanding money for basic security. ES community edition was not a good citizen of the Open Source world.
3. You don't hear this kind of FUD from the PostgreSQL community. Because that is an actually Open Source project with a really good community. Amazon participates in it as a full member of the community: contributing dev time to not just get their features in, but to also review patches that have nothing to do with AWS and of no benefit to them, to even paying for PG evangelism.
4. AWS provides no hosted ArangoDB service.