I think we are sort of saying the same thing at the end of the day, but yes, I actually AM saying that genetics limits the potential to gain one or the other to some extent. The kids who are fast twitch will generally speaking have an easier time with tumbling and quick movement. It's reality. Sure, yes, I see what you're saying. I'm a good example of someone who is naturally fast twitch but built endurance and can run a pretty respectable half marathon. But I am still naturally better at running a 5k. It's just easier to improve my time on short distances.
And yes, the correct form and set is very important too!