Exactly -- nobody (yet) has written any kind of general purpose mapping from rest concepts to relay endpoints. Relay appears to leverage graph db functionality/semantics pretty heavily.
Pete hunt said it should be relatively easy to wire up a RESTful api to flux relay. And since mapping data to complex front ends is hard, but defining restful endpoints is easy, I'd rather design my database to be optimized for the complex front end scenarios. I imagine that you would be able to make a standard REST api from graphql.