Is there logic to the authors assertion past “that isn’t what XML was intended for”? It is a pretty nice data format if you want wide compatibility and schema integration.
The point being, I think the author is arguing you should use the right tool for the job, and XML not being designed for arbitrary data structures makes it not the right tool. Just recently people have shown you can build a raytracing engine in SQL, but if someome was arguing we shall call it SQLCycles and ship it in Blender, I'd definitely have a few objections!