Comment by A_D_E_P_T

Seemed to me that the author was referring to regression to the mean, as another commenter noted.

De novo mutations have a negative effect, to be sure, but it is extremely weak on an individual level. In parents who are extraordinary, the effect of regression to the mean is going to be 20x to 40x stronger than the effect of de novo mutations. For instance, if you have two parents who are both 195cm tall, the regression penalty might be 4cm, whereas the mutation penalty would be somewhere in the millimeters, so a statistically average child would be ~190.9cm. If both parents are statistically average, there'd be no regression penalty and only a vanishingly small mutation penalty.