I don't think I ever brought it up but that was my assumption. Yamato would be the "combatant" so therefore Zoro is something else, and that's by far the most appropriate description for a text box.
Killer is like Sanji as Zoro, the blonde Zoro they've always wanted. We have yet to see the fighting strength emphasis or field command of the real Zoro.
Is killer the vice capitan?