Because as the
guy in your video says, "You want your wrist as stable and straight as possible with all those bones aligned in the best straight line." If you hit with the "stronger knuckles," you have to bend your wrist when turning over the jab to the face or chest.
Hunt demos the turning over the jab as a defensive move. As Hunt's student throws the jab, he turns his front foot and shifts his weight to the back foot while turning over the jab, which brings the front shoulder up protecting the chin. Hitting with the bottom three
or the middle knuckle and the next lower knuckle (not pinky) helps keep the wrist more "stable and straight."
That extreme defense will set up your offense, turning over the jab to turning over the right cross. Turning punches over in training can be used to exaggerate and develop full power, as the
Winklejohn video showed too. So when someone says “CMA has rotation or spirals and boxing does not,” they likely don’t know what they’re talking about. Karate tends not to move their head and not attack & defend simultaneously. So, your alignment may be different when turning over punches.