Many people don't have the misconception that they can compete athletically at the level of olympics. And yet, somehow, a lot of people mistakenly believe they could perform intellectually at that same high level!
I think you run into the barrier much faster in something like sports because the gap is so easily measurable. E.g. it just takes one 100m sprint to see how slow you are.
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.
I'm not convinced. Our brains are well know for their plasticity. Our musculoskeletal and circulatory systems: not so much. The brain is capable of rewiring itself after some quite traumatic injuries, of course the people who go through rehabilitation after a traumatic brain injury do so with the focus of someone whose life depends on it.
I would suggest that most people reading this are capable of Bellard level productivity, if their life depended on it.
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.