Let me explain the problem GraphQL actually solves. FB, as we know, is an incredibly large mobile app, possibly one of the largest in terms of actual code. Here are some realities about mobile apps:
1. Once released into the wild, that app version is out there forever. At app installs in the billions this is a serious problem. So you need something that handles versioning of your API. You need something to force you think about versioning and support old versions as long as possible. Again, I don't see the word "version" anywhere in this post. Versioning doesn't seem to be a problem the author has;
2. There are a lot of common objects in the FB app. Posts, comments, etc. When these objects become sufficiently complex, you don't want individual teams pulling out random fields. This is one thing fragments are for. It allows a specialized team define the object model and allow other teams to pull out a subset of that data.
3. FB uses an in-memory graph database for almost all things so issues like N+1 queries (which are real issues if you're talking to a relational DB) don't really come up. The in-memory database has a layer over it that does things like enforce permissions and privacy policies on reads and writes. GraphQL auth is really a superset of this. So if you're querying an endpoint for a user and their blocked users (one example from the post), you probably won't have auth in your endpoint because that'll be enforced by the data access layer that already exists and be written and maintained by a team responsible for that data.
Some comments here have complained that GraphQL is a technology for an organizational problem. I would counter that by saying all technology ultimately is tied to organization. They affect each other.
The above was all done to avoid things like privacy leaks, which at FB's scale is a real problem. Speaking from experience, a significant amount of computing power and software engineering time is tied to avoid these problems by the teams responsible as well as other teams to try and detect an unintentional leak.
How I see a bunch of people use GraphQL (not necessarily the author of this blog post) is as a 1:1 map to a relational model beneath it. That doesn't make a whole lot of sense. GraphQL probably isn't for you.