As I understood it, he earlier refused the idea of a resource scouting party. You can't really expect anybody to take seriously that this one man is qualified for it unless he has a master's degree in geology as well in addition to everything else. Not that it would make this show any less hilarious.
He was supposed to be on his way to the capital, after being commanded to return there. Yet he decided himself he's not going there. Instead the HQ commander will soon learn Itami took a car, weapons, and food and went who knows where (since only the dark elf knows where). That is not how the military works.