It's true, but that's a valid way to approach the question. The problem is that grandparent will have more misses (people who legitimately believe that the earth is larger than the sun by the assumed metric) than hits (people who think outside of the box by using a different metric).
Actually, a lot of people use the "what's more directly relevant to me" metric. Thus, you have a lot of people who pick "earth" as an answer even though they are not delusional. At least not in this specific way.
