New fields don't constitute the kind of backwards-incompatible changes mentioned in the blog post (adding fields is just as easy with RESTful APIs). GraphQL does help cut down on payload size (no extra fields for clients that don't need them), but if you're making major changes to the shape of your data, you'll still need to write some kind of compatibility layer.
I've done this with GraphQL (major changes to the shape of the data), and I still think it's easier than other approaches.
For example, in those cases where the response object was fundamentally different, I've added a whole new Query field (and Mutation field) at the top level, that returned the new object. Under the covers there was still a bunch of complexity needed to support both top-level query fields, but old clients continued to access the old query, and new clients went to the new query.
Also, two other things I really like: With GraphQL it's trivial to log which clients are accessing which fields (at some point you can just decide to kill old ones). Also, the @deprecated support in the GraphQL schema IDL integrates wonderfully with the GraphiQL tool, so anyone browsing your schema for the first time always sees the recommended latest version that they should use.
