I don't typicall buy appeals to authority when the answer is otherwise readily discernible.
He must hold on throughout the process, unless he completes a football move during the process, such as lunging. That's been clearly stated by NFL officials many times -- the example is right there in the case book. The NFL's explanation, however, was that it wasn't "enough of a" lunge, implying that some lunges aren't sufficient and some lunges are, which is clearly in conflict with the rule book.
Did you even read the other thread? Or the other thread before that?
First, the example you are quoting was in a case book from 2012. It wasn't in the rule book last year or the year before that.
Second, even if it were still in the rule book, it clearly states that if a player is going to the ground while in the process of making a catch, is able to BRACE themselves and then simultaneously lunge, loses control of the ball, it's a catch. Clearly you don't know what brace is intended to mean. Just as you clearly don't know what "football move", "becoming a runner", "being upright long enough" is intended to mean.
It boils down to the following:
(a) secures control of the ball in his hands or arms prior to the ball touching the ground; and
(b) touches the ground inbounds with both feet or with any part of his body other than his hands; and
(c) maintains control of the ball long enough, after (a) and (b) have been fulfilled, to enable him to perform any act
common to the game (i.e., maintaining control long enough to pitch it, pass it, advance with it, or avoid or ward off an
opponent, etc.).
Player Going to the Ground.
If a player goes to the ground in the act of catching a pass (with or without contact
by an opponent), he must maintain control of the ball throughout the process of contacting the ground, whether in the
field of play or the end zone. If he loses control of the ball, and the ball touches the ground before he regains control,
the pass is incomplete. If he regains control prior to the ball touching the ground, the pass is complete.
It's worded poorly and is left to be pretty vague, but the meaning has been and still is this. To have caught the ball, after (a) and (b) you must be in control of your body and able to protect yourself. Still on your feet. On your feet and not in the process of falling. Or on your feet long enough to have been able to protect yourself and in control of your body before you begin to fall.
The reason for this is to make sure that a pass catcher doesn't get blown up with a big hit while he's in some sort of awkward position and then they rule it a fumble. They want the pass catcher to establish the ability to mitigate and protect themselves from that. It has always been intended to be that way.
Now, if a pass catcher is going to the ground while in the process of making a catch, then they most likely don't have the ability to mitigate and protect themselves from a big hit. Who knows how many possible positions they could be in. All of which exposing them to getting blown up. So if they are going to the ground they need to make sure they control the ball through the process of contacting the ground. That eliminates any possible fumbles by a basically defenseless pass catcher.
Now, where I think the point of real contention is in this boils down to this "lunge" reference in a case book from a few years ago. It is NO LONGER in the rule book. But again, the intent was that if a pass catcher is going to the ground and is able to BRACE themselves and then lunge it would be ruled a completion, even if the ball touches the ground. BRACE means to support or regain control of your body and be able to mitigate and protect themselves.
Dez was going to the ground during the entire process of the catch. His lunge was performed while still in the act of falling. The ball hits the ground and comes loose. Incomplete pass.