I read these statistics as containing two trick questions (rot13: cnegvrf naq nobegvba ner obgu gbb pybfr gb pnyy, ernyyl).

I'm inclined to dispute that, but I suspect I'd be implicitly arguing about the definition of "trick question" rather than anything empirical.

I think that asking about Vietnam is addressing a totally different bias than is ostensibly the point of the post; not being familiar with history, and having to draw conclusions based on how that era was depicted, is essentially generalizing from fictional evidence, not anything reflective of our knowledge of public opinion.

Historical questions about public opinion are nonetheless questions about public opinion, and people with a sufficiently good knowledge of something can answer questions about it without drawing on fictional evidence.

Also, I'd bet weakly that the average LW member does better than the general population in guessing public opinion, even when uninformed.

Agreed.