I honestly don’t understand. If this was Williams’ 50 greatest drivers I’d surely have Keke in the top 10, but Mclaren’s? He only drove one season for Mclaren, and didn’t win a race (although he would’ve won in Adelaide without his puncture).
That being said, Prost didn’t trounce Rosberg in 1986. The difference was so great because Rosberg had 8 retirements and 11 DNF’s against against Prost’s sole. In terms of raw speed, Keke was as fast as Alain.