There's no way Xebec was the WSS surely?
I mean if he truly was then I don't really have a problem with it as long as there is proper thought there but in the case that he was then what happened to his title when he was defeated? Moreover albeit a swordsman, Roger for instance has never been referred to as Kengo or Daikengo so how does that work as those titles are correlated to the WSS title. Also merging the heights that Luffy is supposed to surpass with Zoro's dream is a bit off.
Mihawk agenda aside I don't really see why Oda would add this element to the story or what benefit it has going forward.