The problem is that RBY, in addition to the arm behind the back, has clearly been working on his defense/counter offense to the attacks on his 'exposed' side. He does his little lateral rip while holding onto the foot/ankle, torques the knee really sharply (yet subtly), then pulls it up into his nearside cradle he looks for on DeSanto in that position.
Same way he injured DeSanto in carver with it. You can see the adjustments DeSanto has made to avoid getting his knee torqued. In the dual this year, when DeSanto would get in, he was basically sitting on his ankles to prevent RBY grabbing and ripping them. You could hear the coaches go crazy whenever RBY would go to grab that and start his little ripping motion.
Go ahead and go back to watch. Whenever DeSanto is on RBY's right leg, watch how RBY grabs DeSantos right foot/ankle. At times you can even hear the ref go "Keep it legal" which indicates to me that the Brands made it a point to mention to the ref prior to the match.
IMO DeSantos path to victory in this match is to do the things he's been doing against RBY, except focus on doubles, keep the pace of the match high as usual, keep the movement high, work into handfighting flurries, fake shots and come up into upper body ties. Go for a body lock/Merkle type score. If you're going to try to transition to single leg/hi crotch attacks, maybe try from underhook throw by scenarios. Don't go down to the mat to finish, keep your legs clean.
RBY's strategy in these matches is to have stellar defense and get his 1 quick TD off of DeSantos forward pressure. Try to make that 1 flash moment in the match where he gets his TD. Wait for RBY to try to capitalize on your forward pressure and have something waiting for him when he explodes.
Just some of my rambling thoughts.