there is really no incentive to not lie.

Not true. It would probably take at least 20 minutes to fudge all the stuff that has to be fudged. When you're already fatigued from filling out survey questions, that's even less desirable at that time. At best, this would be falling for a Pascal's mugging. True that some people may. But would the majority of survey participants... at a site about rationality?

Perhaps they were merely victim to the Dunning-Kruger effect when self-reporting IQs

They were not asked to assess their own IQ they were asked to report the results of a real assessment. To report something other than the results of a real assessment is a type of lie in this case.

I would again like to suggest that future surveys include questions asking people how much time they spent on average preparing for exams

That's a suggestion for Yvain. I don't assist with the surveys.