In the same way that climate deniers think they know what they’re talking about because they have an elementary school-level understanding of the weather, flat earthers think they know what they’re talking about because they have an elementary school-level level understanding of physics, and antivaxxers think they know what they’re talking about because they have an elementary school-level understanding of medicine, social darwinists think they know what they’re talking about because they have an elementary school-level understanding of evolution. They heard “survival of the fittest” and were convinced that’s all the nuance there was to have about the topic.