His point about the runtime complexity of an API being entirely distinct from how the interface to it's code is exposed (whether GraphQL or REST or otherwise) is fairly obvious, I think.

The counter-argument is that unlimited query complexity makes it far bigger problem, and the author's point is that if you're using it for private APIs with persisted queries, you shouldn't have that problem unknowingly.

Don't get me wrong - I think the takeaway is that GraphQL's niche is quite small, and he's defending exactly that niche. It's not often the case that you can develop an API in a private manner which doesn't undercut higher-order value in the future, as the rise of AWS hopefully made evident.