[austerity]

> This means that there must be at least thousands of IQ alleles to account for the actual variation seen in the general population.

Or that IQ is only governed by genes to a very small extent in which case the rest of the argument goes out of the window.

[compactmani]

You're right that the author (unless I missed it) didn't say what proportion of variance in IQ is explained by additive genetic effects (narrow sense heritability) . I think a quick search would likely answer that.