ummm... maybe because for most golfers who don't have a square face at most, if any, points in the swing, gettting the hands ahead = bad/worse slice?
Flick's view isn't completely wrong, it's just not complete, or put into the proper perspective (i.e., he doesn't explain the open face is the big problem) He does talk about forearm rotation, just not enough about the back of the left hand.
He's talking to the 'average' player, and for them, that view is usually true