IQ heritability within a population is often estimated around 50-70%. Why does that figure say almost nothing about why two populations might differ in average IQ?

D. Because heritability describes variation within a group, not between.

Lewontin's seeds make it unmistakable: split a varied batch between rich and depleted soil, and within each pot height differences are almost entirely genetic while the difference between pots is entirely soil. Heritability is a variance ratio computed inside one population in one range of environments, and the Flynn effect is the clinching illustration.