I think many people are confused about this post. There is no need to believe that nobles were initially "superior", and that their initial superiority was the reason they became nobles in the first place. The increase in "gene quality" at the top, and its diffusion to the bottom, can be understood as the simple result of Malthusian constraints, and high survival rates for nobles (as compared to the harsh conditions of the lower classes).

Imagine you start a society where nobles have the same "gene quality" as everyone else. Some nobles are pretty smart, others not so much. As nobles would have more children surviving into adulthood (due to better nutrition and living standards) compared to the amount of land/titles they could distribute, some of the children of the nobility would be cast aside. The assumption, and I agree it's an assumption (although a rational one in my opinion), is that the smarter children would tend to inherit, and the less capable children would be dispossessed.

Over time, this constant churn would lead to a concentration of "good genes" among the aristocracy, which would trickle down into the lower classes when every generation some of the nobility lost their position.

I believe that this "trickling down" phenomenon has been proven to have occurred in England during the second millennia by Gregory Clark using census data, although I'll admit I've only heard of his work, not read it.