Peyton-Jones is not the only person working on Haskell. In fact, if it were
only Peyton-Jones working on Haskell, the language wouldn't be where it is today.
Yes, the speed of development probably almost always scales sublinearly with the number of people working on it, but beware that perception may also be distorted by the old adage that "the last 20% of the work take 80% of the time". That is, when people are added to a project that has finished the first 80%, it may seem like progress scales terribly sublinearly with the people added, but what is observed may simply be the fact that the same amount of work leads to a smaller perceptible change once a project has sufficiently advanced.