More fundamentally however eugenics is baked into reality. For a simple example look at how well you are when you are handsome, smart, healthy, tall and fit. And look at how you are treated when you are the opposite. I do not want to add any ethical qualifier to this but it is reality.
Eugenics is the most obvious example of the moral failure of utilitarianism, which in the end is really just man's attempt to invent morality for himself. In the end it results in an extremely ugly and selfish worldview. The "unfit" are to be exterminated in utero, the poor are to be prevented or dissuaded from reproducing. The claim is that this is better for the environment or for humanity as a whole, but I say the truth is that it's more convenient for the elite class who invented this evil ideology in the first place.
The pendulum swing of liberalism seems to naturally lead to Eugenics, when it's at it extreme. When I talk with my liberal friends, that's where discussions often end up - "Well, the dumb people are just fucking everything up." Even Sapolsky cites evidence of conservatives being less intelligent - suggesting, ironically, that they're a lesser category of people.
The problem with transhumanism is in the inequality it potentially engenders and the subsequent questions it poses about moral models of responsibility. If you can engineer a better human, aren't you now obligated to accept responsibility for the failings of those you don't improve? You can't control the behavior of the improved, albeit stochastically and indirectly, without implicitly accepting that those you don't aren't responsible.