Your understanding of the meaning of sentence is fine. You have to remember that you are working with a script that is trying to mimic casual American English, but in this case, the script is pretty straight foward. Bart is telling Homer that he is very sure that Piggy is not coming back. Part of the meaning of the line comes from the fact that the sentence before that is "Give it up Dad." That translates to "Give up the idea that Piggy may come back." By stating in two sentences the same idea, it comes across as rather emphatic that Bart is confident that Piggy isn't coming back.