the acid proof weapons may look like metal, but they are actually carved from the alien exoskeleton, the preds probably have a way of carving & polishing the bone so that it appeared as metal, although, aliens are meant to be "bio-mechanical" lifeforms anyway, furthermore, aliens' chitinous exoskeleton is stronger than steel, so it wouldn't easily break during a fight, on top of that, the preds probably have a way of treating their weapons to increase their durability