Nothing would ever be obscurantist for a perfectly rational mind that correctly evaluates every sensory input according to whatever evidence it provides for any logically possible hypothesis.

Not technically true. It is possible to make a perfectly rational mind produce worse predictions about the world by providing it with selected information. This relies on it having insufficient information about your obscuring tendencies or motives. The new probabilities that the rational agent has will necessarily be a subjectively objective improvement but can still produce worse predictions of the relevant aspects of the world in an objective sense.