The site in question lists several alternatives, some of which are widely in use especially in cases where XML falls short: brevity, fast to parse, easily readable... Most of all, calling it the de facto standard is either dishonest or clueless.
You can call me clueless or dishonest, I don't care. I can only share my experience with the topic. You don't have to believe me.
It is in fact the de facto standard in the publishing industry. The "other" format is of course PDF.
Maybe all of this will change when we have more technologies that support formats that can handle mixed content as easily as XML.
None of my customers have ever asked for that. I have not seen a printing house that demands XPS. So I doubt that it plays any role in the market (Germany here).