Very cool! It got me daydreaming about an alternative to html designed from the ground up to minimize webpage bloat and avoid tracking (1x1 pixel gifs, etc.)
XML is a standard format for markup (e.g. HTML). When XML is used where it shouldn't (object serialization) it adds complexity. Here it signals that creating a report is as simple (or at least as standard) as creating HTML. What other format you had in mind?
You can still write xml-compliant HTML; whether it's called XHTML or not is splitting hairs. You still get all the benefits of a very rich toolset for document preparation. I work on a site that makes heavy use of xslt and works with xml-compliant HTML as input/output.
XML and XHTML are two different things. Related, but different. XHTML is no longer encouraged for website markup, that part is true too. That does not mean that XML is no longer encouraged.