This article picks up some good points, but it seems like the main criticism is that Diataxis isn't useful for a programming language because that requires a sequence of lessons. But docs aren't a textbook, if you want to create a learning resource that's great - but that's not documentation IMO.
The textbook aspect of programming languages really is the key idea here IMO. There is something in the textbook example that doesn't seem to line up well with Diataxis's 4 modes of documentation. And IMO it totally does fall under documentation. If writing something like the Rust book is the difference between your project succeeding and failing, then that's the type of doc you've got to focus on. You can't not do the work just because it doesn't line up with some theory of doc types.
