Yeah, the trick here is understanding why the EV of the discarded box gets incorporated into the unpicked box. The reason is that the picked box has been removed from the possibility of being eliminated. By picking whichever box, we have eliminated it from being revealed. So the two remaining boxes are separated by our initial pick from the initial pick itself. These boxes combined have a probability of holding the prize of 2/3. When the empty one is revealed, the two unpicked boxes still together have a 2/3 chance of holding the prize.