Reading back further into the context I see your point. Imagining such an AI is sufficient and Eliezer does seem to be confusing a priori with obvious. I expect that he just completed a pattern based off "AI box" and so didn't really understand the point that was being made - he should have replied with a "Yes - But". (I, of course, made a similar mistake in as much as I wasn't immediately prompted to click back up the tree beyond Eliezer's comment.)