It’s not an inherent law of the universe that if you have to cite a 30-minute video, it means you don’t actually have any cogent arguments. But it does seem to be a law of the Internet. Perhaps that’s for the best, though; it means when the deepfakes arrive en masse, we — or, at least, the critical thinkers among us — will be suspicious already. Let’s hope automatic skepticism of videos spreads before then.
My guess is that the citer is passing on the video because they found it persuasive (it’s usually accompanied with, “You have to watch the whole thing!”), not because it would persuade anyone else or because it illustrates some significant point to consider. It’s a litmus test for community: “I was persuaded by this! You should be too.” I would also suggest that the citer would not claim the video was “persuasive” (they wouldn’t use that term) but “the truth.” At that point, bring in McLuhan. There’s a dissertation lurking here.