The author isn't saying that intelligence isn't heritable. He just showed that we can measure heritability in things that don't exist (the sum of a persons weight and height in some units) as well as things that exist like their actual weight and height. Therefore, the fact that g is heritable doesn't mean that its a thing that exists.

Except I think the author would point out that there isn't just _one_ heritability for things like height because height would be much more heritable in environments where everyone has adequate nutrition as opposed to in environments where nutrition was a matter of luck, for instance.

All of this assumes a certain philosophical veiwpoint on what it means to "really exist" which I don't agree with, but which I do recognize as coherent.