Simple fact trivia won't do anything to demonstrate a person has understood the article, merely that he read it. It's the fundamental difference between functional illiteracy and "plain" illiteracy. This difference shows up all the time when analyzing data because some jurisdiction will only report the illiteracy number while some other will include the functional illiteracy as well.
I think you're ignoring that many people might not even read the article at all. Proving comprehension sounds incredibly hard; proving someone has at least tried would probably be a huge step in the right direction compared to the mayhem which is most comments online.
I rarely read the articles posted on HN. The discussion is frequently much more interesting and often more enlightening. I generally only read the article if the filter of other people's comments suggests that it would be worth my time.
I think I am probably not alone in this.