This is a good advice if you want to hit serves at full speed. But the explanation is wrong.
Racquet not in line with forearm has nothing to do with additional energy. It is a kind of buffer - wrist is not fully extended, its position is more natural at impact, more stable and the risk of injuries is lower.
Watch best servers, frame by frame. They pronate as late as possible, just before contact. Why? Because pronation is not important. They have to pronate because they do not want to hit the ball by the edge of the racquet. They do that as soon as possible, because they want to use as long as possible the main source of energy at serves: arm rotation.
So, why to pronate? Can't we start with the racquet's head in hitting position? Try to do that (carefully!).
We can't. Our wrists are too weak to deal with so huge forces. This is the real source of forearm pronation at serves: weakness of wrists.
LeeD, could you help me, just once? Could you tell me, where can I find "Ignore" button here?