hmm i say its skill, but it really depends.
like my friend is pretty good at no-scoping and he has this method he learned from another person. something to do with like waiting for the cross hair to become small then if they shoot, it always goes straight.
and...
i would say its luck if the person is just running around spraying the rounds everywhere and hitting the enemy somehow. but ive done that before and t NEVER hits soo i dont know about that haha.
so as an opinion, i would say PURE skill

^