I don't think most utilitarians and rationalists accept error theory, or at least most of them say that they don't, and consequently there won't be the same reason for distrusting them. For example, Eliezer calls himself a utilitarian but he still believes that "murder is wrong" is an objectively true statement about the relationship between murder and the abstract pattern which we call "right". And he agrees that it means neither "we don't like murder" nor "game theory doesn't recommend murder."

It may well be true that some people do accept error theory, but don't admit it. In this way they will advance their goals by getting people to trust them. I would guess that you behave that way in ordinary life as well (in your previous comment you said that you can talk and act as if it is objectively wrong to murder.)