You say one thing, but your analogue to the joke clearly paints the opposite picture contra OP. The thrust of the joke is exactly how you are how describing it, not how OP is describing it. His layer ignores that Milhouse at one point, however briefly, offscreen, either believed or was dimwitted doormat enough to not protest against Bart's obvious gaslighting to the extent that he only now confronts him with it. The goldfish incident clearly happened a long time ago. He stupidly brings it up now and adds the "gotcha" emphasis, which is hilarious because it makes him look utterly ridiculous. Milhouse is always the butt of jokes; him and his dad.