At which point you run into a problem of formalization and choice of parameters, which is the same process of ape-brain-based decision-making. A statement that in some sense, decision theory/probability theory math is the correct way of looking at things, is somewhat useful, but doesn't give the ultimate measure (and lacks so much important detail). Since x-rationality is about human decision-making, a large part of it is extracting correct decisions out of your native architecture, even if these decisions are applied to formalization of problems in math.