His hand/arm is so high because it's stuck on top of Sakho's head. By the time the ball reaches his hand he's looking the other way. Let's also not forget that the reason Sakho is jumping from beneath Skrtel is because Kompany pushes Skrtel in the back.
A penalty in this scenario with a group of players all contesting would of been very harsh. Talk of retroactively punishing Skrtel is the kind of shite seen on Blue Moon.
You need to remember that where handball is concerned, intent is irrelevant. If his hands aren't by his sides then it's a penalty regardless of what he meant to do. And if you jump up with your hand in the air at a corner, you're asking for trouble.