Here's the rule:
Prohibited contact against a player who is in a defenseless posture is:
forcibly hitting the defenseless player’s head or neck area with the helmet, facemask, forearm, or shoulder, even if the initial contact is lower than the player’s neck, and regardless of whether the defensive player also uses his arms to tackle the defenseless player by encircling or grasping him
lowering the head and making forcible contact with the crown or ”hairline” parts of the helmet against any part of the defenseless player’s body
illegally launching into a defenseless opponent. It is an illegal launch if a player (i) leaves both feet prior to contact to spring forward and upward into his opponent, and (ii) uses any part of his helmet to initiate forcible contact against any part of his opponent’s body. (This does not apply to contact against a runner, unless the runner is still considered to be a defenseless player, as defined in Article 7.)
The defender briefly left both feet as he made contact with Addison combined with the hit being in the neck area. I can see people's argument that it shouldn't have been called but the ref erred for safety.
The horse collar was questionable but I think the ref saw how far the receiver was out of bounds being pulled back forcefully. The defender should have let go and not completed the tackle. That's what caught the refs attention.