Well he said more POTENTIAL. You can definitely pop a bigger forehand with the double bend position; you just need far more racket head speed to pull it off.
Monfils has knocked the biggest forehand I've ever seen in terms of speed, and uses a double bend. But look at how much energy he has to put into the shot. He's jumping, twisting, and exploding. Then you look at Federer do the same thing (at 90-105 mph), it looks almost effortless. Then you look at Nadal do the same thing at 111 mph. It looks like more effort is used than Federer, but far less is used compared to Monfils. Nadal isn't exploding off the ground and putting every little bit he has into the shot.
Nobody can make it look as smooth and easy as Federer or even Nadal.
A straight armed forehand generates power more easily than a double bend because it's much easier to extend your racket through contact. A double bend is more likely to pronate through contact, creating nothing but spin. Your body has to create the power. With a straight arm, you can generate both easily in whatever combinations you desire.
If Monfils could master a straight arm forehand and put as much acceleration into the ball as he does with the double bend forehand he has now, he could probably be #1 in the world with that shot alone!