Your post deserves a much better answer than what you have received thus far. Unfortunately I am not a physicist and my communication skills are terrible, but I will give it a try. Hopefully someone else will come along and give a much better explanation.
As I understand it, destructive power comes from energy impacted by the projectile. One way to achieve that energy is to have the projectile explode (bombs, artillery shells, nuclear explosions) or just using the kinetic energy (bullets). It is the later type that is being pointed to.
The damage of the kinetic projectile is (I think) based on the mass of the projectile and the amount of force used to propel it forth. Essentially the more dense the projectile and the more force is used to launch it, the more energy it will transfer on impact. Given large enough values, it is possible that a pure kinetic projectile can transfer more energy than a nuclear explosion.
The premise of the post is the assumption that a projectile (rocks and rods) simply thrown from a satelite towards the surface of the Earth, has enough potential energy to achieve this level of destruction.