Here's why it was the correct call.
https://operations.nfl.com/the-rules/nfl-video-rulebook/defenseless-player/
- A receiver attempting to catch a pass who has not had time to clearly become a runner. If the player is capable of avoiding or warding off the impending contact of an opponent, he is no longer a defenseless player
- The intended receiver of a pass in the action during and immediately following an interception or potential interception. If the player is capable of avoiding or warding off the impending contact of an opponent, he is no longer a defenseless player.
Note: Violations of this provision will be enforced after the interception, and the intercepting team will maintain possession.
Inherently, because the player was ruled to have made a football move, and clearly did, he became a runner. At which point a shoulder to helmet hit is not a penalty.