I mean that what I call my 'morality' isn't intended to be a map of my utility function, imperfect or otherwise. Along the same lines, you're objecting that self-proclaimed moral nihilists have an inaccurate notion of their own utility function, when it's quite possible that they don't consider their 'moral nihilism' to be a statement about their utility function at all. I called myself a moral nihilist for quite a while without meaning anything like what you're talking about here. I knew that I had preferences, I knew (roughly) what those preferences were, I would knowingly act on those preferences, and I didn't consider my nihilism to be in conflict with that at all. I still wouldn't. As for what I do mean by morality, it's kinda hard to put into words, but if I had to try I'd probably go with something like 'the set of rules of social function and personal behavior which result in as desirable a world as possible the more closely they are followed by the general population, given that one doesn't get to choose what one's position in that world is'.
EDIT: But that probably still doesn't capture my true meaning, because my real motive was closer to something like 'society's full of people coming up with ideas of right and wrong the adherence to which wouldn't create societies that would actually be particularly great to live in, so, being a rather competitive person, I want to see if I can do better', nothing more.
It sounds like you agree with me, but are just using the words morality and nihilism differently, and are particularly using nihilism in a way that I don't understand or that you have yet to explicate.
It also seems to me that you're already talking about what you value when you talk about desirable worlds.
I have noticed that the term 'nihilism' has quite a few different connotations. I do not know that it is a coincidence. Reputedly, the most popular connotation, and in my opinion, the least well-defined, is existential nihilism, 'the philosophical theory that life has no intrinsic meaning or value.' I think that most LessWrong users would agree that there is no intrinsic meaning or value, but also that they would argue that there is a contingent meaning or value, and that the absence of such intrinsic meaning or value is no justification to be a generally insufferable person.
There is also the slightly similar but perhaps more well-defined moral nihilism; epistemological nihilism; and the not-unrelated fatalism.
Here, it goes without saying that each of these positions is wrong.