Earlier quoted context omitted.
Or variation in intelligence is genetically controlled in ways we don’t understand yet because our understanding of genetics is nascent.
Yeah, sfblah is right. We will be able to predict based on our knowledge of the little molecules inside you at some point, we just can't do it now. The 70-80% is heritable and not via parenting. It is contingent on an environment that provides things like food and air, and the basic idea is that we control for the environment in some simplistic way via twin studies.
It is also shown by the so-called "Flynn effect" [0], which is a substantial rise in measured IQ by over 1 SD in numerous populations during the 20th Century alone. This is comparable to the mean difference in intelligence between individuals, and absolutely cannot be accounted for by changes to genetics.