Genetic determinism isn't so clear, unfortunately. Humans having two legs is highly genetically determined, but that doesn't mean that variance in the number of legs is explained by genes; it's more likely to be environmental factors, like injuries.
Another factor is reversion to the mean. Two bright people are likely to have children dumber than them.
He said correlation, not determinism. It's quite possible for wealthy parents to have smart children on average and yet for any given child of smart parents to be spectacularly dumb. (I know a couple myself, FWIW.)
Something with two legs is far more likely to be human than insect, for example, even if you do have the odd human with only one leg.
Another factor is reversion to the mean. Two bright people are likely to have children dumber than them.