The article's point is, in part, that xz is more like zip or rar than gzip or bzip2, and use of it like those is incongruous (e.g. .tar.xz files). (Its other points are that xz has a number of systematic flaws, in its error detection and some "misfeatures".)
Ah - I presumed that the debian xz-based packages were using it as an archive, instead of the old 'ar'. But I just looked and you are right, the xz support is for the data.tar.xz that itself is still inside the 'ar' archive format. That is odd. I guess they chose this way for simplicity - far less code needs changing in all the .deb file processing utilities.
I can't speak for Debian, but I personally did not realize that xz was as I stated above before reading this article, so I wouldn't be surprised if some people simply didn't realize it.
