I believe a similar research study was done on HN where they asked HN people what their IQ was and actually measured it.

I vaguely recall the average guessed IQ was 120 or 130 while the actual IQ was around 100 which is similar to this study. In a nutshell the conclusion was that people on HN like to think they're smarter than average and like to up vote articles on IQ when in reality HNers are just average.

I can't find the study anymore. But I remember seeing it somewhere.