Here is the portion of his writing that I want to address:
If one is going to argue to for or against any public policy on pragmatic utilitarian grounds, then one has to make interpersonal utility comparisons. Of course you can also make non-utilitarian arguments, as when people oppose government intervention on "natural rights" grounds. But it is not easy to convince the broader public without some sort of utilitarian argument.Here, Sumner is using a sort of modus ponens argument. He states that if one is going to argue for or against any public policy on pragmatic utilitarian grounds, then one has to make interpersonal utility comparisons. He then goes on to show why these pragmatic utilitarian grounds are the best way to approach public policy and, thus, by affirmed the antecedent, he can assert the consequent.
The conditional statement that he is asserting seems sensible, though there are reservations that one might have about it. Let us assume it is true. Instead of using modus ponens, let us use modus tollens. By negating the consequent, we can negate the antecedent. We have plenty of reasons to believe that interpersonal utility comparisons are not possible, namely by pointing out that utility is ordinal, not cardinal. Murphy gives a fuller account of these reasons in the link above.
Given these reasons, we can deny the consequent of Sumner's conditional and, thus, have a reason to deny that one ought to use pragmatic utilitarian grounds to argue for or against any public policy.