Are you a groomer? This isn't causing much discomfort to the dog. Brushing pulls as well, and it's necessary to get the knots out. Pulling the fur like this is actually almost necessary to see the severity of the mats, because the comb shows how close the mat is to the skin. As I said, my grooming manager told me this when I started (it's ok to pull if you're brushing correctly, it doesn't hurt them), because I am typically incredibly gentle with dogs. In grooming, you can't be that gentle. The entire grooming process causes more discomfort than what the dog is feeling in this video. I'm not trying to be rude, this is just not causing much discomfort. Judging by the dog's body language, he's perfectly content lol. He's literally smiling for half of it.

Source: dog daycare experience and bathing in a salon under groomers who've been at it for years.