It misses the point: if you split the universe in two identical parallel universe, but then take the same individual and in one universe train them 2h per week, and in the other universe, train them 8h per week, do you really think, whatever the test is, the second one will not perform better?

This is the point of training: the more training you have access to, the better you do. If it was not the case, then the notion of school itself as a way of training people to be able to think by themselves will not have any sense.

And that is just training. Even with the same amount of class hour, kids who don't have to worry about take care of their siblings, of the house chores, or of even having access to decent relaxing conditions will get higher score even if they are in fact less smart.

There are studies that show how heritable intelligence is. Very.

It is quite common people say, that something is only a correlation and not causation. But if you can point to a common denominator, that has been shown multiple times, to have a massive effect, it's not likely to be just a coincidence. Genes are this common denominator. Society and habits (for example, protestants vs catholics) are another.

Things that consistently impact the whole population are not just a random process that picks: "You will be clever, ugly.", "You will be pretty, sporty, but will be dumb." It's always genes, society and habits.