I misread the point they were trying to make, I took the 5 std to be a sign they were pulling from general population IQ. On second reading it’s clear that is not their intended point.
In any case the resulting intermediate distribution from selection criteria bias would be a Chi distribution (I think - it has been a while.) 5 standard deviations on a normal distribution is 175 IQ or top 1/700K which is insane.
A few additional biases, Physics majors have higher average IQs, and not everyone smart enough to do the math has the desire or the opportunity.
The point I was making is that IQ score has been normalized, when imagining the intermediate distribution of IQ it would make more sense to look at composite probabilities of factors that led to the IQ score and consider the probability that those same factors lead to a sufficient innate ability in math.