If it hadn't it his arm, it would have hit his body so I don't see what he's preventing exactly. Look, yes I am biased... but this isn't just me saying it, it's pretty much every ex ref who is in the media and knows the rules. They have all to a man from what I have seen said it's not a penalty.
Sky absolutely love this "luck" angle at the minute, they got a ref to go through all the decisions and he's said they were pretty much all correct. In fact, there were more wrong against than for if I remember rightly.