OTOH, using a vague definition isn't a good idea either, if you want to know something about what Less Wrongers believe about the world.

I had no problem with the question as worded; it was polling about LWers confidence in a specific belief, using terms from the LW Sequences. That the particular belief is irrelevant to what people who self-identify as various groups consider important about that identification is important to remember, but not in and of itself a problem with the question.

But, yeah... if we want to know what proportion of LWers self-identify as (e.g.) atheist, that question won't tell us.