No doubt that's true at the limit, and it explains why a person can train as hard as humanly possible and never become a professional racer, while another doing the same trianing wins the Tour de France.

However, the vast majority of cyclists are no where near their theoretical limits, and virtually all of us could be faster with more, and/or better training, rest, diet, etc.

Chalking up being slow to genetics is a rationilization. If you're slow, its because you choose not to do the work to be fast. Which can be a perfectly valid choice, but it's a choice.