What you say is certainly true if the utility monster is simply exaggerating. But I understood VM to be discussing someone who claims offense where no offense (or negligible offense) actually exists. Or, someone who self-modifies to sincerely feel offended, though originally there was no such sensitivity.

But in any case, the real source of the problem in VM's scenario is adhering to an ethical system which permits one to be exploited by utility monsters - real or feigned. My own ethical system avoids being exploited because I accept personal disutility so as to produce utility for others only to the extent that they reciprocate. So someone who exaggerates the disutility they derive from, say, my humming may succeed in keeping me silent in their presence, but this success may come at a cost regarding how much attention I pay to their other desires. So the would-be utility monster is only hurting itself by feeding me false information about its utility function.