Seems like you're looking for reasons not to give it.
You said the hand on the back wasn't enough for him to go down and then say you can't even tell if he was going down before the leg contact.
You see how this is illogical?
Not really.
The contact is supposed to be enough for the player to go down in the way they did. I can clearly see the slap on the back and obviously that isn't going to make Ronaldo's legs go from under him like that, so it's irrelevant to the penalty call.
The
only thing the penalty can be given for is the contact with the legs, because that's the only thing that happened that might have justified Ronaldo going down as he did.
If he was just striding forward and Zouma tackled him, that's a penalty. However, if Ronaldo was already going to ground before there was any contact then obviously it becomes more difficult to argue the contact caused that reaction.
I'm just finding it hard to see whether he started going down before or after the contact with the legs.