And a soft piece of sponge is also a weapon when you force it in someone’s throat. If you define “weapon” like this, almost anything is a weapon. You lose the distinction between a bow that was designed for killing and a brick which was designed for building.
But more importantly - if everything can be a weapon when used as such, then saying that an interstellar capable spaceship is a weapon says nothing about spaceships themselves or interstellar travel itself.