Ok. So his arm was in a hugely unnatural position, increasing his area and making it harder to cross. Maybe if his arm was at his side, the cross would have gone where his arm was blocking and led to a goal? Having his arm up highlighted the issue and made it inevitable the decision was given. If he was playing with his arms not blocking crosses, maybe he would have got away with it.If his arm was at his side it would still have hit his arm. Why is it different that his arm was raised when it hit it?
Was a terrible decision.
But, even if I give you that the initial contact was not a foul, the ball was still live and deflected off him towards the penalty area where Liverpool players were waiting. He then brought his arm down and, in so doing, controlled the ball along the whole length of his arm and finished with a little basketball dribble off his hand to stop it moving towards goal and deflect it down into a bounce. You cannot watch football and claim that it isn’t a penalty if a defender starts bouncing a ball off his hands in the penalty box!