No, -desu was explained at the beginning of the episode, you have a whole 20 minutes (or more than 1 episode, I can't remember) to get to that point, so the jok

No, -desu was explained at the beginning of the episode, you have a whole 20 minutes (or more than 1 episode, I can't remember) to get to that point, so the joke worked perfectly fine as it was originally