If you're able to drive the conversation like that, you can get information about IQ, and that information may have a larger impact than race. But to "screen off" evidence means making that evidence conditionally independent- once you knew their level of interest in physics, race would give you no information about their IQ. That isn't the case.
Imagine that all races have Gaussian IQ distributions with the same standard deviation, but different means, and consider just the population of people whose IQs are above 132 ('geniuses' for this comment). In such a model, the mean IQ of black geniuses will be smaller than the mean IQ of white geniuses which will be smaller than the mean IQ of Jewish geniuses- so even knowing a lower bound for IQ won't screen off the evidence provided by race!
Huh, sure, if the likelihood is a reversed Heaviside step. If the likelihood is itself a Gaussian, then the posterior is a Gaussian whose mean is the weighed average of that of the prior and that of the likelihood, weighed by the inverse squared standard deviations. So even if the st.dev. of the likelihood was half that of the prior for each race, the difference in posterior means would shrink by five times.
Here's the new thread for posting quotes, with the usual rules: