I dont think anyone is saying genetics are the only thing that matters but if you are willing to manage the fish to try to maximize the size, then yes genetics play a huge roll.
I look at it this way you have 4 options:
1. Bad/No management + "normal" genetics = high probability for smaller fish
2. Bad/No management + "great" genetics = medium probability for small fish
3. Good management + "normal" genetics = medium probability for big fish
4. Good management + "great" genetics = high probability for big fish
