you're making things up. Suggest you watch a replay because clearly you haven't. He didn't "let" them have a shot! He was already in the process of blowing his whistle when they took the shot. That is not letting them have it. You've become obsessed with this two chances and lost focus on what actually happened. Let it go!
Look 20s, I normally have respect for you but I'm also aware of your history of digging trenches and keep digging when in discussions. However, I will do you the courtesy of one more reply before I put you on ignore. I'm not willing to waste any more of my life explaining the laws of the game to you. If you want to carry on to have the last say or persuade anyone of your views, then that's your prerogative. If people want to believe you, then more fool them imho.
Unusually for the laws of the game, the scenario on Tuesday is pretty well covered. So let's go back through it.
Under the advantage rule, the referee is entitled to wait and see how play might unfold. There is no definition of "wait" other than he is instructed to make a quick decision. The referee is instructed to weigh up two options - is it better to give the foul or let the team carry on. However, when there is a chance involved, he needs to weigh up whether the chance that would emerge from playing on is better than stopping. In this case, he had to weigh giving a penalty (i.e. a direct shot against the keeper from 12 yards) versus the ball going to a wider angle and shooting with no goalkeeper.
The whistle was in the referee's mouth as it should be in this scenario. He is instructed to make a quick decision, and in the speed of play, it needed to be very quick. He should have been and indeed was ready to make the call either way. Frankly, where his whistle was is irrelevant to anything other than to say the referee was clearly prepared.
He let (allowed or didn't blow the whistle, take your pick) play continue and Peterborough had a chance to score. You may think that chance was hard but there is nothing in the laws that says if the chance is hard, the referee can ignore it. That is the consideration the referee is instructed to consider. If you don't think shooting on target from an angle when there is no goalkeeper is not a chance, then I really can't help you.
The chance was missed and the referee only blew when the chance was missed.
The laws of the games clearly state (black and white, and not open to interpretation), that (unlike other advantage scenarios where no immediate scoring opportunities arise) the referee cannot allow two chances i.e. he can't let them shoot and miss, and then give the penalty.
The facts are the referee thought the foul warranted a penalty, allowed them (by not blowing his whistle) to have a chance and then gave the penalty afterwards. The laws of the game instruct that he shouldn't have done that.
For the record, it would have been quite harsh on Peterborough and maybe the law should be reviewed to allow the referee more flexibility but that wasn't the question. Should he have given a penalty after Peterborough missed their chance. The answer is No.
Also for the record, I was in line with the Peterborough player who took the shot. I thought at the time he should have scored but his shot made it easy for the defender.
And also for the record, given you're earlier unfounded accusation, no I don't write this cos I am bitter about the referee. He actually had a pretty decent game. They make mistakes and you will not find many examples of me criticising them throughout my history. I have done refereeing and I also know referees including one who does National League. I have a lot of respect for the effort they put in, the preparation they make, and how difficult it is only to see the criticism they get. They have a thankless task and have to make decisions in a split second. However, in this case, he got it wrong although I can fully understand why.