[Dimensio]

By keeping the “unfit” alive to reproduce and multiply, eugenics proponents argued, modern medicine and costly welfare programs interfered with natural selection –the concept Charles Darwin applied to the “survival of the fittest” in the animal and plant world.

As human inventions, modern medicine and welfare programs are themselves natural. They do not "interfere" with natural selection, they simply alter the environment variables in which natural selection operates.

[flevit]

then eugenics/genocide is also competely natural part of evolution just 'environmental variables'.

I take that wording to mean, they saw it interfered (egocentric none the less) by selecting (sematics???)against their population leading them into extinction...and took active role to be the 'superior' race.