How much variation is explained by genetics versus environment depends on the range of genetics and the range of environments you're looking at. In the US you've got the the full range of normal h sapiens diversity but all children get to drink pasteurized milk, eat iodized salt, use indoor plumbing, and go to school. If you look at a country like India where that isn't true you'll see a much more pronounced impact of shared environment on IQ.

And it isn't that IQ is all genetic, it's that it's half genetic, half something we don't know how to measure, and very little shared or family environment.