In other words, even if this is completely correct, it doesn't disprove relativity. Rather, it disproves either relativity or most versions of utilitarianism--pick one.

It seems like all it shows is that we ought to keep our utility functions Lorentz-invariant. Or, more generally, when we talk about consequentialist ethics, we should only consider consequences that don't depend on aspects of the observer that we consider irrelevant.