It's actually very common for commentaries to be recorded prior to a show airing or a film being released, too. Which is sort of a shame because they never get to weigh in on the fan reaction in those commentaries. But yes, it's fairly standard practice if the product is finished in time and in this case, it was because of reasons Danny gave.
Ahh... I supposed I was spoiled by the majority of Volumes 1-4 and about half of Volume 5. If the world revolved around me, they wouldn't do the commentary for an episode until after it airs.