Taken out of context, my statement is too general, yes, and does look like the dismissing-theists-as-idiots thing, yes.

What I was saying was intended to be understood as "Those who accept theism can't be trusted to have correctly reasoned about the specific nature of the theos, because the very same influences that caused them to be theists are going to be inducing them to defend a specific theos whether it makes more or less sense than the alternative."

Given the tendency of people to put things in domains, I will, in fact, (reasonably) trust what a Vatican astronomer says about the Andromeda Galaxy, or a Creationist nuclear engineer says about Three Mile Island, et cetera. But the existence of a theistic deity and the nature of a theistic deity seem closely-enough related, domain-wise, that I won't trust a theist to tell me he's rationally evaluated whether God is One or Three, rather than rationalized it.

And, from my outsider perspective, I'm just not going to guess whether trinitarianism is more complicated, or if it just seems more complicated when you don't know what problems it solves. In physics, I trust that if the more-complicated-seeming answer of relativity didn't give better answers than the simpler-seeming Newton, physicists wouldn't use relativity. In theistic theology, I can't trust either proponents or opponents of trinitarianism to be giving me a rational evaluation as to whether the Trinity is an overcomplication or, overall, simplifies things.