I think what Eliezer is arguing against is "I don't know" being used to shield information that allows the speaker to privilege hypotheses that are obviously wrong if we had taken basic background knowledge into account. For example: "I don't know if there is a teapot floating around the asteroid belt." The teapot hypothesis is absurd if we base our priors on background knowledge, but the previous sentence makes it sound more plausible because it hides this information.