Before WT sent for the transition, TKD should already have had his near knee pulled up against WT's hip to block the stepover to mount. His arms should have been either wrapped around WT to hold him close and smother his punches or else framing on WT's neck to keep him away and upset his structure while TKD shrimped out to reach guard, reverse position, or regain his feet. Since his legs were disconnected from WT and his arms were just covering his face, he had no way to see or feel WT starting the transition to mount, which he would have needed in order to time a bridge or shrimp for an immediate escape.
Once he was mounted, he needed to shoulder walk backward so that WT's elbows weren't in his armpits, then use a body hug + bridge or bridge + knee bump to force WTs hands to the ground. Then the first option would be to hug WT's body close with one arm while wrapping one of WT's posted arms with the other, trap the ankle on the same side as the trapped arm, then bridge. (There are other options when that one is shut down, but that's what I teach my students first.)
From the looks of it, TKD guy either had zero ground fighting training or at most he got a few tips before hand but didn't spend enough time practicing to remember even the basics under stress. His reaction was pretty much exactly what we saw from pure strikers in the early days of MMA who had never encountered ground grappling. In this day and age, I don't see why anyone would knowingly enter an MMA match (even a casual, amateur one like we see here) without learning some ground fighting basics. Not saying he had to learn how to do armbars and such, just the basics of defending himself and getting back to his feet.
