Is 'impermanence' skillful means?
Well in one sense impermanence is ultimately true. In the other sense though it's just a concept, an idea, used to make a point. The objective of the point is to preempt grasping for things as if they were permanent, and the purpose of non-grasping is prevention of suffering. But even prevention of suffering is merely one of many motives that leads a person to enlightenment, other motive for example being perfection or conceit. In this sense everything is just a skillful mean inasmuch as it pushes some people in the right direction.