As I remember Mathew Syed's book he largely based his claims on skill based activities (i.e. playing music, games, maths tricks etc.). I think he still accepted that there was still a physical limitation to how well anyone could preform.
I would argue the physical: skill ratio is larger for running than for a game, so genetics plays a greater part. Of course the more you run the more efficient a runner you will become but there is a limit. Not everyone has with the perfect muscle and bone development nor the best genetic variants of metabolic enzymes, etc.
I think most people know others who quickly become impressive runners off little training and others who work their butts off for little gains. It's near on impossible to say what one person can achieve. You can make as many excuses and explanations as you want but as there are so many factors to making the perfect runner the only way to find out for sure is for that person to try.
Edited: 31/10/2012 at 00:41