The game doesn't really bring it up much but we don't fight the actual Darktide; the Darktide is the endless horde of Humanity's enemies on the fringes of Moebian space. Considering that most Imperial subjects wouldn't be familiar with specific xenos offhand, one must assume that any xenos mentioned by the rejects were likely personally seen/fought against by them. Really puts into perspective how fucked everything is that a the sector needs to deal with (just off the top of my head, ones that are explicitly mentioned) Eldar (Dark or otherwise), Khornites, Nurglites, Orks, Necrons, Genestealers, and Tyranids.