ive never seen that footage. thanks
for the OP, ive also been taught that suburi should help develop kokyu;
kokyu, particularly in the left hand, should facilitate the raising of the weapon, as opposed to muscle.
doing this is what helps in doing many reps, otherwise the arms get tired quick.
also, as the strike lands horizontally, ive been taught that one should practice rolling the index knuckles over top (also related to kokyu, i guess?), which helps one feel 'on top' of things--something that i find is related to the moment in ikkyo-yonkyo where uke's balance is taken and they are made to go down; if they are trying to bounce back up, it is this feeling of being 'on top', through the grip, that helps one maintain control. i find it develops a strong sense of center.
also, fwiw ive always thought that doing suburi strictly for repetition and without watching these things closely is a waste of time. im a big fan of iwama weapon-work, but i can see how there is a very fine line between suburi that teaches one something vs. people just waving sticks around and at each other