World's strongest creature is above any other world's strongest. So if mihawk is strongest swordman, zoro beating WSC makes him strongest among all. it doesn't need to be a sword duel if the feats of another swordman surpass the feats of present wss.
it's like mihawk beating vista to become wss. if another swordman,zoro, beats kaidou, even if kaidou is not swordman, zoro is the strongest, not mihawk.