I can understand the explanation given in the clip, but it isn't the decision I would have given. I would have gone with an attacking scrum for the knock-on. If you are going to award a PK every time something like that happens then there aren't going to be many knock-ons that don't result in a PK.
I've watched it again and revised my opinion ... Blue 10 was closer to the action at the time of the knock-on than I originally thought he was, so yes, Red did take the action to cynically prevent loss of possession.
Totally understand that. Whenever I view video clips like this I try to make a judgement on my first viewing in real time, as that most closely replicates what I would have to do in a real match scenario. I am unlikely to ever have the luxury of a TMO and generally don't have ARs as part of a team of three either. I think it is highly likely that I would have called the scrum for the knock on in real life. With the benefit of replays it seems clear that there is a case for a PK against Red as Blue 10 is lawfully trying to gain possession of the ball and is (unlawfully) prevented from doing so by Red, but a call either way by the referee would be perfectly understandable.
If nothing else, I think he would be guilty under Law 10. Having been called for an infringement he is then cynically obstructing Blue 10 from taking advantage.
Not necessarily cynical in every instance, but I think you could argue that it was cynical in this situation and at this level of match where the defender knows that loss of possession will almost certainly result in a try against his team.
