The problem is that the line between what has felt like a "moral" preference and what has felt like some other kind of preference has been different in different social contexts. There may not even be agreement in a particular culture.
For example, some folks think an individual's sexual preferences are "moral preferences," such that a particular preference can be immoral. Other folks think a sexual preference is more like a gastric preference. Some people like broccoli, some don't. Good and evil don't enter into that discussion at all.
If the error theory were false, I would expect the line dividing different types of preferences would be more stable over time, even if value drift caused moral preferences to change over time. In other words, the Aztecs thought human sacrifice was good, we now think it is evil. But the question has always been understood as a moral question. I'm asserting that some questions have not always been seen as "moral" questions, and the movement of that line is evidence for the error theory.
If the error theory were false, I would expect the line dividing different types of preferences would be more stable over time, even if value drift caused moral preferences to change over time.
The line between "truth" and "belief" is also not stable across cultures.
I think there’s a confusion in our discussions of deontology and consequentialism. I’m writing this post to try to clear up that confusion. First let me say that this post is not about any territorial facts. The issue here is how we use the philosophical terms of art ‘consequentialism’ and ‘deontology’.
The confusion is often stated thusly: “deontological theories are full of injunctions like ‘do not kill’, but they generally provide no (or no interesting) explanations for these injunctions.” There is of course an equivalently confused, though much less common, complaint about consequentialism.
This is confused because the term ‘deontology’ in philosophical jargon picks out a normative ethical theory, while the question ‘how do we know that it is wrong to kill?’ is not a normative but a meta-ethical question. Similarly, consequentialism contains in itself no explanation for why pleasure or utility are morally good, or why consequences should matter to morality at all. Nor does consequentialism/deontology make any claims about how we know moral facts (if there are any). That is also a meta-ethical question.
Some consequentialists and deontologists are also moral realists. Some are not. Some believe in divine commands, some are hedonists. Consequentialists and deontologists in practice always also subscribe to some meta-ethical theory which purports to explain the value of consequences or the source of injunctions. But consequentialism and deontology as such do not. In order to avoid strawmaning either the consequentialist or the deontologist, it’s important to either discuss the comprehensive views of particular ethicists, or to carefully leave aside meta-ethical issues.
This Stanford Encyclopedia of Philosophy article provides a helpful overview of the issues in the consequentialist-deontologist debate, and is careful to distinguish between ethical and meta-ethical concerns.
SEP article on Deontology