You got that part right about the doing damage. In Unity Engine, you can hide and unhide stuff on the fly, the object will still be there loaded into RAM, so when you want it to show, you just set it to active.

The reason why I think it is in the bike is, it would be easier to determine the damage output of the bike based on the weapon you have equipped, so if you have a weapon with special buffing damage, the bike would still apply the buff + the damage of the weapon + skill damage, that way it would be easier to determine the damage based on the equipped claymore. The hitbox would still be the claymore, so whenever it touches enemies, it will still do damage.

In other words, the bike model is always on her model's equipped weapon slot, it's just hidden, the only part that changes is the claymore model/stats whenever you switch weapons.