I just invented XDF, it is exactly like XML, but it can be used to represent arbitrary data structures. Do you have any objections now?

XDF looks cool! Some of the design choices look pretty inefficient and arbitrary for a data format though, have you thought about rebranding it as a markup language? =] /s

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!

It would be a rather bad design. It would not survive a good design review.

That is only valid if the XDF standard has been published by the W3C

This doesn't make any sense. W3C doesn't restrict/define use cases of XML, it defines structure and semantics thereof; using it as-if it is an XDF document is perfectly ok, just like you can use XML data for your DASH stream etc... It's a structure on top of XML structure.

In reality it's actually fine as a data format for lots of use cases.

Documents are unstructure data, markup provides structure to unstructured documents, so xml turns documents into data.