[0xcde4c3db]

I think you're missing pjscott's point. Sure, a fixed set of genes determines the maximum intelligence across varying environments, but it's equally true that a fixed environment determines the maximum intelligence across varying genes. How is one or the other "the" maximum?

[MagnumOpus]

You can measure and modify the environment for a growing child/youth with relative ease; but you cannot modify genetic makeup at all (for now). This makes the distinction a valuable one.

Think of a car analogy: Genetics is the engine's size and type, environment is the fuel/tyres/lubricant that were added. Quite possible to get very bad performance out of a car with no engine oil or fuelled with diesel instead of petrol - but once these "hygiene factors" are addressed, the maximum performance is determined by innate factors.