Sure, sometimes it is, depending on your goals. For example, if you start a religion, modeling certain moral proposition as true is useful. If you run a country, proclaiming the patriotic duty as a moral truth is very useful.

I meant model::useful, not memetic::useful.

I don't see how this answers my question. And certainly not the original question

It doesn't answer the original question. You asked in what sense it could be true or false, and I answered that it being "true" corresponds to it being a good idea to hand it off to a powerful genie, as a proxy test for whether it is the preference structure we would want. I think that does answer your question, albeit with some clarification. Did I misunderstand you?

As for the original question, in a world where utilitarianism were "true", I would expect moral philosophers to make judgments that agreed with it, for my intuitions to find it appealing as opposed to stupid, and so on.

Naturally, this correspondence between "is" facts and "ought" facts is artificial and no more or less justified than eg induction; we think it works.