how do you randomly sample high iq? by definition the population would have had to have taken a test. so already, even if the study looked at every known high iq individual in the world, it would still be biased -- for those with academic backgrounds, psychological histories, criminal backgrounds, and whatever else leads one to take an iq test. nb i am not advocating for this particular study nor their methodology.
You would have to sample a lot of people (yes, there is a selection bias in the people that participate) and give all of them IQ tests.
What you meant to say is that IQ tests are biased, not that “any sampling method is biased”. IQ tests are biased, but statistical sampling methods can be unbiased.
Are there any widespread test given in schools that are proxy IQ tests? Mensa accepts older ACT and SAT scores as they consider the current versions to be knowledge tests but older versions to be intelligence tests. Not sure if this is still the case but at one time all high school students in the State of New York were required to take the SAT prior to graduation. While not representative of the entire population, it would have been a good representation of the high school student slice of the population across the entire state. Perhaps that would have been good enough for some uses.