The book is wrong. Evolution from the start discredited eugenics or any notion that there were superior races. How could there be if all types of humans had been evolving for the same amount of time?

People supported eugenics because they were racist, not because the theory of evolution ever taught them it was a good idea. People would have manipulated the ideas to fit racism, but it wasn't the cause, and studying it, even then, would have led to the opposite conclusion that the racists were trying to make.

People already knew about selective breeding, selecting against deformities in people has been present since ancient times. Darwin did not introduce this concept.

Also, the idea that Christian morality stood against eugenics at the time was laughable, since the both the British Eugenics Education Society, and the American Eugenics Society counted top clergymen among their members, and modified their message to appeal to Christians.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4001825/

Meanwhile eugenics was being consistently denounced by biologists and anthropologists, and they were especially concerned with the unscientific ideals eugenicists were spouting in order to propagate their message (such as picking undesirable 'traits' that weren't even biological in nature ie committing crimes)