The feature is still draft and only made version 1 in June 2009 (http://xmpp.org/extensions/xep-0166.html). I don't actually track changes in XMPP so I am open to correction on this point.

Around the time I actually wrote the article (many months before it was originally published in .net magazine) there had just been a libjingle release and I was told at the time it was a new thing to have this available to 3rd party developers.