Because we're talking about the same thing in a different language. "Genetics* only mean the instructions used to build the structure of our intelligence. They're not wrong at all, our determinant nature is the result of the structure of our neurons which has a fundamental configuration predominately determined by genetics.
No, it doesn't invalidate it because the twins could be fraternal, having separate DNA, or that could have had such extreme differences in environment that it drastically changed the DNA from the shared base. Excluding those, though, and you can see the deterministic nature of life shining through.