Given that it's an essential/definitional issue, I would have preferred that the author at least showed awareness of the differences between languages intended for marking up text and languages intended to represent data in a structured way.

Back when XML was first being developed, I was really anticipating having a standardized, easier-to-implement successor to SGML (which was hampered by its complexity and by the cost of the ISO standard) in the text markup space. It was that disappointing it ended up filling the vacuum in the space of serialized representations for for structured data, then getting rejected when it wasn't quite as suitable for that as alternatives such as JSON.