The Bare Bones XML specification

Well, in a way, he is right. The XML specification does not address separation of content from format, or most other design decisions, and XSL-FO is in XML document syntax and is about page formatting. Yes, but.

XML was written as the ‘good parts version’ of SGML. It is a short document that concentrates on document grammar and well-formedness; DTD grammar and syntax, validity constraints, entities, processors, and the like. It assumes that you want to know how to define a markup vocabulary and says nothing about:

Document-based XML is assumed, by large numbers of its practitioners, to include:

But the XML Recommendation mentions none of that.