Agreed. Any referee willing to put their time into something that helps other referees learn the trade has to be a nice chap.
The problem with subjective decisions, especially something as 50-50 as this one, is that it's a little disheartening to see an answer marked as wrong when it's definitely not wrong in law. I haven't actually done the quiz and not sure what was considered as the right answer but I can guess. To make the point on this occasion, had the attacker just stopped in his tracks and throw his hands up after 'the hold' instead of swinging them, all of us would have agreed it's a pen. The only reason he swang it was because he was being held and it was impeding him. I can also see why a DFK and yellow is credible here.
Or put it another way, neither pen nor a DFK sould have been recommended a review by VAR.