After reviewing the video, could it be that the reversed grip was only used against foolish opponents. In the plate shown by Ben the guy on the left is showing off or acting like an idiot. So the guy on the right is realizing this and sets up in what looks to be an indefensible position, a taunt if you will. After the guy on the left stops playing with is sword perhaps he is going to go for the obvious attack, an zorn or something easily defended by the reverse grip guy on the right is in.
I think the plate is showing us that when you meet an opponent who is an idiot trick them. The guy on the right also looks pleased with his opponent, maybe I'm way off... is there any text to this?