To me, it looks like DJ made the initial effort, then decided a desperation dive to get ONE more ball back wasn't worth the toll it would take on his body, only to lose the following point because Fed had position.
I'd reckon Kiteboard is bigger and stronger than Fed. Certainly outweighs Fed by 40 lbs.
I have little power in my backhand overhead, but placement can trump power if the placement is good, and you hit the unexpected shot.
Notice also that n the last couple years, lots of drop volley winners. You don't need to hit hard to hit a winner.