I think Johnny accepted what Sami was saying way to fast. (Especially when he added, "Yeah, we don’t need her.") I don’t believe a little kid would "get" what was going on after being that upset that quickly. Although, the little trooper sure played his part well.
I wonder if he is memorizing those lines or if they’re taking a break between each one and feeding them to him. Whatever, he’s sure doing a great job. I loved the expression on his face when he was pouting into the corner of the couch and telling EJ he hated him. Very good emoting and delivery for such a little tyke.