From what I can tell and what I have read around the internet the final play call almost sorta kinda maybe makes sense.
It was an RPO and based on how Denver lined up, Ben selected the "Pass" part of that. I've watched it a bunch since yesterday and if the snap gets there clean and the NT doesn't randomly get pushed back about 3 yards, AB was initially open. When he first cut the route inside, he was "open" - a narrow window but it was there. In fact, he still looks open and has leverage on the CB when the ball is thrown. AB has a step on Harris and as long as Ben leads him to the far post, he hauls it in no problem. Just no one expects the NT to kinda rear up and snatch the ball.
All in all, when Ben had to handle the bad snap and delay the throw, a bunch of mess happened. With the biggest being that Ben rushed the throw and threw it low and behind where he should have. He should've thrown it 5 yards into the stands.
Video here:
https://www.youtube.com/watch?v=sbvrwo-2IIs
Watch 11:48-11:56 -- if Ben would've put the ball towards the O and N in Broncos, Steelers tie the game.