I wouldn't take that result too seriously. If everyone posting on LW had an IQ of 140+ it'd suggest LW posters exclusively came from the top 0.4% of the IQ distribution or so.

The entry requirement for MENSA is 148 on the Catell IQ test. Mensa only requires the 98th percentile. The selection effect could be on which IQ test result they wish to report. I certainly report the thing that sounds better whenever in such a situation. If people are mislead by me reporting an IQ that is through the roof that is their fault for taking such an ambiguous and meaningless number like "IQ" seriously in the first place.