Ah yeah I saw that video before, so bad. If I saw someone belaying like that I'd immediately call them out and tell them to hold the brake strand. Lucky this was even caught on video to prove what the error was. It's disturbing to see how his right hand still appears to just hold onto the gri-gri while the climber falls (rather than grabbing the rope). Inexcusable IMO, especially for someone who isn't a total beginner. There's no "people make mistakes" caveat here, that was straight-up dangerous technique, like driving a car with no hands on the wheel or with a blindfold on.