It's a representation of data.
XML and JSON should be grouped together. I
I'd agree, but you also have to say the same thing about HTML.... As someone who is in software development, I cringe when people say they can "program in HTML"...
But the article was about "programming skills", not languages, and being able to represent data as XML (either by hand or programmatically) is a skill.
And you can cross the line with XML with XSLT.