Agreed. 10 years ago, I could still pretend I'd be able to come close to his productivity if I "just set my mind to it." Today (being older, slower, and having a family), I can only be humble.
Whereas with intellectual work it's often hard to assess the gap, not least because the further you are from closing it, the less understanding you'll tend to have of how hard the remaining parts are.
Sure, let's say 20% of it is genetics, 80% of it is still putting in the work.
I would suggest that most people reading this are capable of Bellard level productivity, if their life depended on it.
As the parent of a 3 year old, I know that it’s at least older than 3 XD