That conclusion really doesn't follow from the stated proposition. The proposition is that doing sprints will make you smarter than you are now, not that it will make you smarter than someone else who doesn't do them. The only conclusion about Usain Bolt that you can reach from the stated proposition is that he's now smarter than he was before he started sprinting. </pedantry>
I read the point as "doing a high speed exercise makes your brain have to work at a faster rate". Sprints being the example chosen as increasing the brain processing speed. Which is quite interesting because there are other activities that require high speed mental abilities. Playing an instrument in a thrash metal band for one. According to this theory, a member of Megadeth would be a faster thinker than a classical musician. Interesting thought.
My point is that you can't draw conclusions about two different people this way. If playing speed metal is a good mental exercise, then the only thing you can conclude about Dave Mustaine is that he's smarter now than when he started playing. You can't conclude that he's smarter than anyone else, no matter what activities they participate in. The classical musician very well may have started out at such a high IQ that no amount of mental exercise will allow Dave to catch up.
I am not sure about the heavy metal / classical music approach. There are many classical music that are very challenging by themselves, even more than some heavy metal songs (which usually are 'play the same note at light speed'). I've heard of an author whose name escapes me now that composed songs that weren't suposed to be played due to the sheer difficult of them.
