If X doesn't add anything to Y, it could be because the two terms are virtually synonymous. The difference between the words would be notational or somehow trivial.

Now you seem to be saying that "happiness" doesn't add anything important to "utility" not because the words are synonymous, but because what happiness does add to utility is irrelevant to economics. That's different from saying that "happiness" doesn't anything at all to "utility," though. And it's quite dubious, too, since of course the moral questions have an impact on economic systems. They don't have an impact on scientistic economics (as opposed to the classical kind), but the standards and self-conception of that value-neutral form of economics are questionable.