If all you mean by "sincere" is not explicitly thinking of something as deceptive, that seems right to me, but if "sincere" is supposed to mean "thoughts and actions can be well-predicted by utilitarianism" I disagree.

I was focused more on the first meaning of "sincere." I think that utilitarian's abstract "far mode" ethical beliefs and thoughts are generally fairly well predicted by utilitarianism, but their "near mode" behaviors are not. I think that self-deception and akrasia are the main reasons there is such dissonance between their beliefs and behavior.

I think a good analogy is belief in probability theory. I believe that doing probability calculations, and paying attention to the calculations of others, is the best way to determine the likelihood of something. Sometimes my behavior reflects this, I don't buy lottery tickets for instance. But other times it does not. For example, I behave more cautiously when I'm out walking if I have recently read a vivid description of a crime, even if said crime occurred decades ago, or is fictional. I worry more about diseases with creepy symptoms than I do about heart disease. But I think I do sincerely "believe" in probability theory in some sense, even though it doesn't always affect my behavior.