My father learned to cook in the catering corp’s of the RAF. He also learned how to jump out of planes and potentially kill people. It was a gag line bit it didn’t break the film. It might have worked better if Finn was pulled out of his sanitation duties by phasma because he hadn’t seen active duty yet. He hadn’t killed yet. Which would be psychologically vital for an army of brain washed zombies. Maybe Finn is still brainwashed and was programmed to infiltrate the Resistance by showing empathy and is now a sleeper agent of some kind.