This doesn't give context at all. Dembele was just completely beaten. He then made contact with Di Maria from behind when there was no chance he was playing for

This doesn't give context at all. Dembele was just completely beaten. He then made contact with Di Maria from behind when there was no chance he was playing for ball. Sure the referee didn't have to call penalty but it was fair he did. A more obvious interference could have resulted in a red card.