Saying "you can object to eugenics on moral grounds, but you can't prove that it doesn't work" is advocating for eugenics. Any notion that eugenics could or could not "work" is built on the assumptions that there is a correlation between the methods of eugenics and their stated outcomes, and that said outcomes could potentially be desirable.

What's more, while eugenics certainly uses the language of evolutionary science, it is directly contradicted by this science. Eugenics movements, such as in the early 20th century in the US, were built on the claim that intervention was needed to prevent dysgenics, the proliferation of "bad" genetic traits. This is nakedly unscientific. "Survival of the fittest" means the organisms that survive the most are, by definition, the most fit.