I think this philosophy only reasonably applies behind the public-facing API gateway. So the GraphQL API server wouldn't be part of the microservice graph that you're trying to make into a polytree (you also wouldn't consider the client-side software to be part of this graph). You can use GraphQL delegation or similar to move more responsibility to the other side of the line.

The only alternative I can think of is to have a zillion separate public-facing API servers on different subdomains, but that sounds like a headache.