The thing is, you don't need to know in detail how a car works to see that car A is faster than car B or that that self-driving car crashes less than that other

The thing is, you don't need to know in detail how a car works to see that car A is faster than car B or that that self-driving car crashes less than that other one. Similarly, you can tell how much of intelligence is genetic for a given environment (and the "for a given environment" bit is forgotten by so many) by comparing clones (identical twins) to non-clones (non-identical twins). Additionally, sometimes even if you don't understand about how anything happens you can still influence the result. E.g. you don't need to know anything about protein expression and brain development to breed more docile foxes. (Not that I would recommend trying breeding experiments on humans)