It's just how the game has changed. A few years ago there used to be highlight reels of monstrous, concussion-causing hits, and it was a great defensive play to lay out a helpless receiver with the crown of your helmet.

A few years ago this would have been a great play. Now it's against the rules, it just wasn't called in this case. Keep in mind that Gallman lowering his head isn't relevant--leading with the helmet is targeting even if he'd hit the ball-carrying arm.

The way they're supposed to call it on the field is consistently--it should have been called targeting if it was helmet to helmet, and in your hypothetical it should be called targeting if it was helmet to arm. The point of the rule is to remove this play from the sport altogether. Now obviously, it's still a "great play" if there's no penalty for it, as it caused a turnover and took out one of Clemson's best offensive weapons for the entire game all at once. So in execution it's still a crapshoot.

But the theoretical answer to your question is that such a move is never allowed and should always be flagged, by rule, no matter what the ball carrier does with his head just prior to the hit.