Ethical subjectivism is also discussed in the Stanford Encyclopedia of Philosophy.

(I like this quote from that article, btw: "So many debates in philosophy revolve around the issue of objectivity versus subjectivity that one may be forgiven for assuming that someone somewhere understands this distinction.")

You may be right to say that my use of "utilitarian" is different from how it's conventionally used in the literature; I'm pretty unfamiliar with the actual ethical literature. But if we have people who have the attitude of "I want to take the kinds of actions that maximally increase pleasure and maximally reduce suffering and I'm a moral realist" and people who have the attitude of "I want to take the kinds of actions that maximally increase pleasure and maximally reduce suffering and I'm a moral non-realist", then it feels a little odd to have different terms for them, given that they probably have more in common with each other (with regard to the actions that they take and the views that they hold) than e.g. two people who are both moral realists but differ on consequentialism vs. deontology.

At least in a context where we are trying to categorize people into different camps based on what they think we should actually do, it would seem to make sense if we just called both the moral realist and moral non-realist "utilitarians", if they both fit the description of a utilitarian otherwise.