IQ tests do not have to be culturally biased. The Mensa admission test (basically two standardized IQ tests) are not biased to any particular culture, do not require that you memorize "facts," and do not require that you be an ace in mathematics. You can be great at math and remember tons of minute details, but you still may have an average IQ.
Real IQ tests are not things you can study for.
I do agree that intelligence is only one of many factors that may cause someone to leave the org. And there's probably no way to quantify exactly how much it actually affects a person's ability to "break free." But there's no denying that intelligence will help someone see through the Watchtower's lies.
In the congregations I have attended, most of the people were not what I would call "smart." It's probably because the WT discouraged college so much; a very low percentage ever attended college.