The announcers often make the rules seem more confusing than they are. In this case they did a good job of not confusing a perfectly legal play. There are a couple of ways that it could have been a penalty if events were slightly different. "Helmet to helmet" as quoted on broadcasts is widely misrepresented and really a combination of multiple other rules.
In this case if Cooks was still making the catch then Tedric could have been flagged for "Illegally launching" at a defenseless receiver under rule 12.2.7. However, Cooks "had time to clearly become a runner" so this does not apply. Once a player is a runner it is completely fair game to leave your feet in an attempt at making a tackle or hit. Similarly, if Cooks was defenseless then Tedric could have been flagged for contacting him in the head under rule 12.2.7 regardless of whether he made contact with his shoulder or even foream.
Once a player is a runner then the defender is allowed to go low or high in order to bring them down. However, the tackler is not allowed to use their own helmet as a weapon. 12.2.6 forbids using "any part of a player’s helmet or facemask to butt, spear, or ram an opponent" it is a penalty under 12.8 if a player "lowers his head to initiate and make contact with his helmet against an opponent". The important part of 12.8 is the word "initiate" and given that Tedric made initial contact with his shoulder it does not apply. Also, in order for Cooks to be flagged under 12.8 the official would have had to feel that he lowered his head to "initiate" contact which in this case he clearly did not.