HGH is the one thing that comes into the equation later.
The claim is that people can squat 500 or deadlift 600 on the FIRST attempt ever and so on.
As for the Arnold claim, sure he was more muscular after one year, but he was probably also on more dianabol than other people in ten years.
If someone squats 500 on his first try ever, he had probably been doing lots of sports all his life before.
It's not genetics IMO when it comes to a certain level. Sure some people naturally run faster than others, but genetics only go so far. Set up a trained genetically inferior runner against a not trained genetically gifted runner, and the later will be the loser.
If, after 10 years of training, the difference between genetically gifted and not gifted still exists, i doubt it.
Modern science concludes that genes in the human body react to certain triggers. This means you can turn genes "on" and "off".
In this whole case, and with all professional athletes, drugs come into the equation, so it's basically all bullshit anyways. A drugged up genetically weak person will always be stronger than a genetically strong person.