I cannot comment about the video, as I don't know the rest of the video.
But here's something I found in my martial arts studies:
Japanese arts punch with the first two knuckles (mostly),
Chinese arts with the middle two knuckles (mostly - or perhaps better said: empasize those to keep the entire fist in line).
Both will tell to punch with the fist straight in line with the wrist: this mainly means with the 2nd knuckle straight in line. Still there's the above difference of explaining which knuckles to use.
Then I started comparing the hands and knuckes of people in Hong Kong: when making a fist their 3rd knuckle sticks out much more than with Westeners, where the first knuckle sticks out more. Talking about bringing Darwin to the punch
Even some teachers in HK realised this and didn't tell Western students to keep the fist flat with 2 middle knuckles, they just let them use what others call 'karate fist': so, in conclusion: as long as it works best for the student (or you).
Hope this helps your confusion, Daoust.
On the other hand, the position of the thumb is different to between Japanese and Chinese arts, did you know and do you know why?