We don’t know that for sure. When Rig asks where the cytonic hyperdrive is so he can try to fix/study it, M-Bot points him to an empty compartment within the ship. Maybe Doomslug’s species was used by humans for FTL travel by putting them in that compartment and exploiting the slugs’ natural abilities. Doomslug could easily have made her way into the compartment between Spin leaving M-Bot and the final dogfight. Or maybe not, maybe the slugs are a substitute for the defect; pilots without the ability could instead make FTL jumps using the slugs.
My guess is that Detritus is one of the known planets, maybe the only planet, on which Doomslug’s species exists, which is why Gran-Gran’s mother took the Defiant fleet there. I’ve also been thinking that the slugs are how the Krell find human settlements, because the slugs are attracted to people with the defect. If that’s true, then Doomslug could have been protected from being discovered by M-Bot’s stealth technology.