[ Usopp is also looking for a little explanation on this event, as he wanders around the party, not really aiming to do anything... It doesn't make sense to him why they're throwing a party in the first place. A robot warden is a robot warden; no matter which one it is, it's not like it's going to free them - they're still stuck in jail.
Either way, he catches familiar sight of the cook, and is happy to see him again. He's just completely ignoring the last time they met... If he thinks about it, it becomes real, so we're just pretending it never happened and greeting Sanji with all smiles. ]