I don't think they're concluding "Fat people aren't exceptionally intelligent" or "Fat people aren't as smart as lean people".
What they found at the core of the study was that something in the fat cells was, in some way, contributing to decreased cognitive abilities.
So going along the lines of your initial thought, we can entertain the idea that "Newell, Torvalds, or Wozniak would have even higher cognitive performance than they already have if they exercised everyday and reduced their store of fat cells". That's more in line with what the article was suggesting.
What they found at the core of the study was that something in the fat cells was, in some way, contributing to decreased cognitive abilities.
So going along the lines of your initial thought, we can entertain the idea that "Newell, Torvalds, or Wozniak would have even higher cognitive performance than they already have if they exercised everyday and reduced their store of fat cells". That's more in line with what the article was suggesting.