Let's conveniently forget to mention the PI call that wasn't called against the Cowboys in the Detroit game immediately prior to the Dez catch game. You just made the exact case the original OP constructed. Dallas advanced in large part due to that call, but obviously since it favored the Cowboys, it was the correct call!
I will give you the holding call. Grabbing the jersey is almost always a penalty - but not always called. The PI call I disagree with. From the video behind Hitchens it looks like interference, but from the side you can see Hitchens it not making contact. He doesn't have to play the ball if there is no contact. The picture shown in the video shows the TE making contact with Hitchens but there is space between the two players. It use to be a penalty to get in the face of the receiver and block his view of the ball but that rule was changed a long time ago.
It is very possible the ref from the wrong angle called the penalty but the ref who saw the play from the side convinced him to pick up the flag.
From the rule book:
Acts that are pass interference include, but are not limited to:
Contact by a player who is not playing the ball that restricts the opponent’s opportunity to make the catch;
Playing through the back of an opponent in an attempt to make a play on the ball;
Grabbing an opponent’s arm(s) in such a manner that restricts his opportunity to catch a pass;
Extending an arm across the body of an opponent, thus restricting his ability to catch a pass, and regardless of whether the player committing such act is playing the ball;
Cutting off the path of an opponent by making contact with him, without playing the ball;
Hooking an opponent in an attempt to get to the ball in such a manner that it causes the opponent’s body to turn prior to the ball arriving; or
Initiating contact with an opponent by shoving or pushing off, thus creating separation.
Note: If there is any question whether player contact is incidental, the ruling should be no interference.
I highlighted the first act and the second. The first part says there has to be contact and it has to restrict the opponent's opportunity to catch the ball. Hitchens did not make contact until after the ball arrived. Contrast this with the penalty called on the Eagles in the Dallas game. The defender ran into the receiver preventing him from getting back to the ball. That is PI. had he not made contact it would not have been PI. The defender is allowed a trailing position against a receiver.
The second part of the rule, playing through an opponent, is exactly what Slay did to Diggs. Even if he was playing the ball he cannot jump on the back of the receiver to do it. Remember when Schoonmaker was tackled as the ball arrived on the goal line? Blankenship did the same thing and was not called. These non-calls were huge and it is what happened when the Eagles played Miami too.
It is pointless to argue about a call from years ago, but it is important to know and understand the rules so we can better understand the inconsistencies and bad calls we see today.