If the ball was out of play, it wouldn't be a penalty. It wasn't out of play, as is clear from the replays from the right angle. So the fact that he wasn't going to score is not relevant. And the fact it was a genuine attempt at a tackle doesn't stop it being a foul and a penalty.
Oh dear:
Agree with your comments. My first thought was the ball was out of play so irrelevant whether there was contact but they later showed that it was still in play at the time of contact so definitely a penalty.