Not to speak for lessdazed, but what I understood them to be saying is that when I argue against a proposition P solely by pointing to the consequences of believing P, I am implicitly asserting the truth of P. I would agree with that.

I would say further that it's best not to implicitly assert the truth of false propositions, given a choice.

It follows that it's better for me to say "P is false, and also has bad consequences" than to say "P has bad consequences."

That wasn't how I saw the context here, because of the statement "I do not agree". Also, no consequences were enumerated. "I agree with the facts as stated, but think phrasing them this way has bad consequences," is a fine way to argue against a presentation of ideas.

I am very suspicious of obscuring truth in the name of positive consequences, of applying only or mostly first-order idea utilitarianism.

It's perfectly fine, for me at least, but I prefer moral objections to be specified more clearly than "I do not agree", which seem more appropriate for the disputing of factual statements. I discuss this in further detail in a comment of mine above.