The difference between initiating a low plunge from the jet combo (N1CJP) is literally 1 frame of input. At 8 frames between your jump and your plunge input, you do a low plunge, but if you wait 9 frames, you will do a high plunge.
Xiao's vertical displacement is super quick. If you want to do the jet combo, you need to get to the point where you get a high plunge, so the video is a good reference to know when you fail/do it correctly. It is also better to be late and high plunge than it is to be early and low plunge.
Players should try to time Xiao’s ult to when an enemy hits the floor so the enemy may receive dmg from it. Random plunge ATKs most likely will NOT hit the enemies. Also players can save their primogems if they were planning to roll for Venti for plans of CC for Xiao. If you are running both Venti and Xiao then alternate ults as you go. When Xiao's Q is up then Venti's isn't and vice versa.