I may be wrong, but I believe that would only apply to kinetic energy. Directed energy weapons don’t depend upon kinetic energy, but rather thermal and electromagnetic effects. They also don’t rely upon projectiles, so no mass involved, the “m” in your equation..
Yes, directed energy weapons depend on getting high power radiation (various types and tech) on a target. My comment was based on the topic of rail guns. Rapid acceleration of projectiles. Smaller mass at higher velocities can prove immense amount of energy on targets. Rail guns have lots of technical issues to still overcome. Barrel lifespan is a big one in previous versions but I’m sure it’s getting better.