You are mistaken about how the maxim of relevance works. The maxim is part of Grice's Cooperation Principal, which basically states that people make certain assumptions when they communicate (the four maxims) and that when those maxims are violated the hearer assumes that the cooperation principle is still operative and the violation was intentional. The speaker is therefore using the flouting of the maxim to create an IMPLICATION.

I love (in a largely non-sarcastic fashion) that someone got this out of ketchup.

Originally I had real sympathy for the OP's desire to make their own ketchup and not be challenged about why, because I've done things like that purely for the hell of it and I don't see what's so odd about it. But all the references to high fructose corn syrup turning people into mindless zombies (except it really doesn't! I'm not saying it does, over and over, or anything!) and the pointless wanking kind of killed that.