Nonsense
I did a quick calculation of the kinetic energy of the collision, It comes out to be the equivalent of just about 4.4 tons of TNT- A hefty amount to be sure, but I cant imagine ANY military use for this....
Obviously if we can hit a comet with that kind of accuracy we can hit the earth as well.
So what?! hitting Earth with that relative velocity would be pretty damn hard to arrange and would be impossible on short notice- You have to get that velocity somehow... and any reasonable sized projectile would just burn up in the atmosphere? WHY go through all that trouble when you can just send a few steath bomber missions at a target...
It is just silly...