The elbow definitely moves in some of these clips but I don’t really think it’s the source of the motion. The motion path that you can see in the clips is not one the elbow can actually create. That large, swoopy vertical motion — the elbow doesn’t go that way. And two, if I do these motions at a super slowpoke speed like 200 beats per minute sixteenths (humor intended), there is no elbow motion at all. Also if I do the motion with a gypsy flex, like Joscho’s strumming technique, there is no elbow motion.
So I think the elbow motion is either me being new / awkward at this and just firing all available muscles probably harder than I really need to. Or, it’s incidental small motion of the elbow joint as a result of the wrist muscles firing. Technically, the wrist muscles do attach on the upper arm. So in theory they can create elbow motion. But they attach right at the end of the bone so it doesn’t give them very good leverage. That may explain why you only see this small elbow joint motion when the wrist muscles are firing at max, and only in certain postures — i.e. death metal posture instead of gypsy.
That’s all just a guess. The practical question is do you have to do something with the elbow to make this motion work? For me, no. Learning to do this motion is very much about trying to tap the hand very fast at the guitar body. If the elbow moves, great, whatever, but I think you want that fast / easy hand tapping feeling first and foremost.