^ Mbappe didn't thwart anything.
Mbappe just done what he has to complete for his team.
Tomasz Kwiatkowski did. Szymon Marciniak did. Simon didn't see a penalty, yes, ball hit elbow but there was no way a running player could prevent his arms from being there and, crucially, the ball hit his arm AFTER rebounding from body. No unnatural arm extension means no pen. Don't know how VAR convinced him otherwise but he's been taken off duty already. Rebound to hand not pen. It's already understood but soon a rule change incoming to England recognising how ridiculous those pens are.
Maybe Simon did not see the situation well and did not think the ball hits the player's arm initially without VAR. I agree with you that it's so ridiculous that with VAR, with slow motion effects, they together considered it is a penalty for PSG. The key point is the ball hits the player's chest before bouncing to his arm. It is a condition to refuse the penalty for PSG. One more condition to refuse it is the arm is very close to his body and it's natural too.
The penalty is like a gift from UEFA for PSG because they don't want to see PSG absense for round of 16. Simon just executed that mission from UEFA and that situation gave him a chance to do that task.