To quote: "Ironically, Fisher was proven right, albeit in a very limited way: such genes [that increase both the tendency to smoke and the tendency for lung cancer] do exist."

The actual issue was not that (Cornfield wrote a paper in 1959 showing the effect was too small). It was that Fisher continued to repeat one finding in one study, despite that said finding had not been replicated in new studies (namely, lung cancer patients described themselves as inhalers less often than the controls), and continued to obstinately ignore all the other research coming out. But it was only 3 years between Cornfield and Fisher's death in 1962, so perhaps Fisher simply did not have time to change his mind.