IF we are to take the shinai as to what they are SUPPOSE to be, ie: live blades, then whatever the Judoka did AFTER they received the cut from the Kendoka is irrelevant because it “never happend”, you certainly cant counter a strike the chopped off your wrist by striking back with that same arm.
Granted that the way kendoka hit with shinais have ZERO to do with how to use a live sword, nevertheless the point is that they represent a “live blade”.
I know that when I was learning the Yagyu Shinkage-ryu system of Kenjutsu we did some grappling VS the live blade and it was pretty clear how dangerous that was IF the guy with the sword knew how to counter the grappling.
Now we can go deeper and see how well a sword cuts VS armour and how that effects grappling and so forth, but that is not really relevant to the video.
Fact is the judoka did well for guys “playing with sticks” with very little prior experience.