The article is certainly contradictive, but I've decided to let it stop with the first sentence:

The correct behavior for an aggregate concatenation query is undefined.

The article then goes out of its way to present scenarios where it may work after all. You should keep in mind that the article was originally published when SQL 2000 was the most recent alternative, and there was not any alternatives.

It's important to understand that just because something works in one specific test, that is no guarantee that it will always work, unless there is documentation to say so. And here the documentation clearly says "undefined".

From SQL 2005 there is FOR XML PATH('') which has a well-defined behaviour. Nevermind that the syntax is clunky and non-intuitive. But this is the way to go if you need to build concatenated lists. (As long as you are not dealing with binary data.)