Superman wasn't referring to his powers being amped by the motherbox, he was referring to the upgrade he got when he was revived from what they thought to be dead. When he came back, all of his powers were stronger than they were before. The only thing the motherbox did was monitor his internals, repair damage, try and counter the poison in Doomsday's claws, etc.. His strength upgrade was from resurrecting himself from dead, as it were.