AAT is very specific: if you are absolutely certain that someone else is rational, then your posteriors and their posteriors must be equal.

You need to have common knowledge of each other's estimates, common knowledge of each other's rationality, and common priors.

I have edited my post to remove the reference to AAT per your and JGWeissman's advice.

Sure. But you're telling me that two healthy individuals are equally efficient at converting resources into happiness? What evidence is there that the Brahmin is not ten times as capable of happiness as the Untouchable?

In Carter's paper he was discussing a hypothetical pleasure wizard so good at converting resources into happiness that it was better to give it all the resources in the world and let everyone else just have enough for a life barely worth living. It seems unlikely that such an extreme pleasure wizard exists, although it's quite possible for there to be some variation among people considered "normal." The psychological unity of humankind provides some evidence against extreme pleasure wizards' existence, although it's far from conclusive.

Preference utilitarianism makes things more complicated since someone may be inefficient at producing happiness, but efficient at satisfying some of their other preferences. However, since being alive and healthy are nearly universal preferences, I think that it's still accurate to call someone with severe illness a reverse pleasure wizard, even if you value preference satisfaction rather than happiness.

Even if you're right and Carter was mistaken in stating that pleasure wizards don't exist, that doesn't alter his main point, which is that equality is valuable for its own sake, and therefore it is immoral to give all resources to pleasure wizards, no matter how efficient they are.