I looked at the play again, and didn’t see anything different. The key here is that the receiver is, by definition, defenseless. That means any forcible contact against him, whether it involves the helmet or not, must fall into 1 of 3 categories, or it’s a foul for unnecessary roughness. The 3 categories of legal forcible contact are:
Contact occurring during a legitimate attempt to make a play on the ball.
Contact initiated with open hands and both arms extended.
Contact occurring in the course of a tackle, where the defender wraps both arms around the receiver.
In my opinion — and I stand ready to be corrected if a different angle shows otherwise — this is forcible contact against a defenseless receiver that doesn’t fall into any of the 3 categories of legal contact. Therefore, it’s a foul.