You're arguing for magic. If brain function does not follow from our physical makeup, then where exactly do you think it comes from? Sure learning is like exercising a muscle and can shape your brain, but environment affects muscle growth too via exercise and nutrition. Would you deny that your genetics place an upper limit on your achievable strength?

Sorry to say, there is already plenty of evidence of direct genetic impact on cognitive ability that you can see with your own eyes, even ignoring the twin studies that all but prove that cognitive functions are largely heritable. For instance, down syndrome, Tay-Sachs disease and Leukodystrophies.

The scientific case is in: given comparable nutrition and a standard education, your intelligence as compared to your peers is determined by your genetic makeup.

This says nothing about your work ethic or your determination of course, and thus what you can achieve if you put in enough effort. Intelligence is merely a metric stating that given the same amount of work expended on cognitive problems, those with higher intelligence will do better on average.