That's a bit harsh. However I checked out the video and didn't like what I saw. When someone holds as described it may be hard to turn a knife blade but it makes it easy to escape the hold. Plus if it was a sword---Oh dear!
I hate to see a "kamiza" built of concrete blocks occupying the mat area. The unfinished walls also lend a sense of "hollowness" that Chris reinforces with a lot of the kind of thing you describe. Grabbing higher up the arm may appear to provide some measure of distance from the knife but it also provides quite a distance between the grasper's thumb and fingers, making escape that much easier. It looks good when your personal uke is making those ineffectual "struggling" actions, but in reality, the knife-wielder's arm will be free in an instant, making not only the defender's arm but his torso, abodomen and throat all easy to attack. And for that reason, I seriously worry for the students (that and the concrete blocks and wooden planks in the mat area).