2 things... 1 for each side of the argument.
i think some people may be getting PI and holding confused. I have only seen it twice, but iirc it was not PI.
The ball being uncatchable doesnt matter. Kaep could have thrown the ball away - it would have still been holding.
contact is allowed for the 1st 5 yards. Seeing that the ball was perfectly on the 5 yard line, it makes for an easier call on when the contact must end. Again, IIRC the holding starting just before the end zone and ended approx a yard or 2 in. Its not like he held him on a crossing route, where more of the holding calls tend to be called. He called him on a vertical route which is a tad harder because you tend to give them a little bit longer than 5 yards of contact on a vertical.
I hate to see a game end like that. I would have hated if they called it though. I think I lean about 60-40 to should have been called def holding.