Even though it shows up as "It isn't" in the captioning on the DVD, I think he says "T'isn't (because he likes to talk in a colorful way).
I never understood the hatred for this episode. Clever plot twist, amusing characters, delightfully absurd moments, fun ending. What’s not to like? Just relax and have some fun!
I saw it again less than a week ago, and I'm still fond of it.