It turns out that genetics is vastly more complicated than the old "gene = expression" model. By the time you've added epigenetics, environmental control of expression, proteome vs genome, and all kinds of other details, you get something we're barely beginning to understand even now.

"neurons = intelligence" looks like another version of the same thing. My guess is neural nets will turn out to be useful for certain kinds of preprocessing, just as they are in the brain. But GI is going to have to come from something else entirely.