You’re not giving them enough credit. Surely they knew what projectile weapons were; they’d just come up with something better. Undoubtedly, the gun to them was a primitive as a catapult or a sling or something. The difference in anatomy—which dictated the difference in design—was probably the only thing that kept them from recognizing Kate’s weapons. No doubt if it had been a quadruped-friendly gun they would have figured it out already.