If either person is using a weapon it is, by definition, no longer hand to hand.
If you look into the subject a little from a real world perspective you will find that it is far from "not hard at all" to disarm a person with a knife.
The gentleman in the photo is a police officer and, I believe, a well trained use of force instructor who went hands on with a guy with a knife while wearing body armour. Inside it's effective range, a knife is up to 3x as lethal and much more effective than a handgun.
If you ever need to "disarm" somebody with a knife who intends to do you harm, I highly recommend that you shoot them first, if at all possible.
I have no opinion regarding the specific incident in the video. I lack the necessary information to form one.
Edit: I see the photo was removed. In retrospect it was pretty graphic.