Humans don't make decisions based primarily on utility functions. To the extent that the Wise Master presented that as a descriptive fact rather than a prescriptive exhortation, he was just wrong on the facts. You can model behavior with a set of values and a utility function, but that model will not fully capture human behavior, or else will be so overfit that it ceases to be descriptive at all (e.g. "I have utility infinity for doing the stuff I do and utility zero for everything else" technically predicts your actions but is practically useless.)

You say that if humans don't implement utility functions there's no point to reading Less Wrong. I disagree, but in any case, that doesn't seem like an argument that humans implement utility functions. This argument seems more like an appeal to emotion, we are Less Wrongers who have some fraction of our identity connected to this site, so you want us to reject this proposition because of the emotional cost of the conclusions it brings about. Logically, though, it makes little sense to take the meaningfulness of Less Wrong as given and use that to reason about human cognition. That's begging the question.