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.
IMO JSON is less readable for this kind of document. XML is more explicit (at the cost of being chatty) but it's actually good for markup. For example nesting is easier to visually manage with XML due to tooling and explicitness of closing tags.
JSON is good for smaller objects you normally use during coding where you do not want the format to get in your way. Here I actually do want a very explicit format as it's the main thing.
