My conclusion is that Marc is being a bit sloppy here. Like 75% of the Y Combinator partners, I have a Ph.D.---and I think graduate (and, indeed, undergraduate) degrees are overrated. But given the small percentage of Ph.D.s even among technical people, it would be very surprising if most people having a high impact had them. More relevant is to ask whether Ph.D.s have relatively more impact than those without doctoral degrees. I don't know the answer, but I think it's a more interesting question.