So can a Knight only use their aspect in weapon form?

Nope, the key word here is ‘Serving’. Using, Weaponizing, and the like are very much specific interpretations of Knight powers in certain contexts, limiting them to just using their Aspect as a weapon would go heavily against the versatility of Classpects! Also if they could only use their Aspect as a Weapon, it’d leave them with no utility for when they’ve completed the game and are ready to claim their spot as Deities of the new World.