Tom had already mastered electromagnetic cannon technology.
However, electromagnetic cannons used for interception and those used for offense are two different things.
Offensive electromagnetic cannons prioritize destructive power; their projectiles must be as large as possible and as fast as possible.
As for the rate of fire, it's certainly better if it's higher, but the requirement isn't as stringent.
Conversely, defensive electromagnetic cannons require projectiles to be as small as possible—they only need to alter the trajectory of an incoming electromagnetic cannon projectile by even a single centimeter.
After a long journey, this minimal deviation will cause it to stray significantly from its original trajectory, preventing it from hitting the spaceship.
This doesn't require much kinetic energy, so why make the projectiles so large?
In fact, making them smaller allows for carrying more.
