Kyle's "functional mobility" over Bertans can be argued to no end, up and down and honestly, it will mean nothing, in the end.
I think Kyle's very good defensive movements are more from his high IQ and anticipation. He makes movements with his hands and arms that have nothing to do with his "mobility".
He "allows" players to get past him so he can maintain a backside blocking angle...
He knows how to anticipate a pass and use his arm length to do good things.
That kid is awesome on D, until he is overwhelmed athletically.
Bertans has enough mobility to keep guys in front of him, but I don't think he has the "arms" (to put it simply) to disrupt like Kyle can.
Maybe in a year, or two (if he's a film rat) Bertans can build that kind of defensive anticipation, but he's just a rookie right now, bouncing about and doing his try-hard, more than his cerebral.
Kyle just has it, for anticipating what's going on, on the court- no matter how slow he moves.