I don't think it's ambiguous: the post suggests rationality means maximizing utility, utility is happiness, therefore rationality means maximizing happiness.

The question then arises: is the amount of focus that Americans place on acquiring material resources (instrumentally) irrational? Three possibilities occur to me:

(A) The very activity of acquiring material resources is a terminal value for most people. People would be less happy if they focused less on acquiring material resources, not because they find having the material resources fulfilling but because they find the practice of acquiring the material resources fulfilling.