Think that the main misconception about this WSS thing is that it is a tittle that someone give to Mihawk, its more like a epithet that people started to call him the more famous he become and the more impressive the duels he take part were.
Don know if in english the words "tittle" "epithet" and "nickname" have the same meaning. But seems that in some things the translation from japanese to english have a lack of words, that made hard for english only speakers to cach what is being said, its kind of like the same that happen when they translate bushi/samurai to swordsman in some mangas for lack of better words. For example, here in Brazil the official version use the world "alcunha", from the Arab "al-kunya", that dont have a english translation, to Mihawk "World Greatest Swordsman" and use "titulo", that means tittle, for his official tittle as one of the "Seven Privatieers of the Royalty" and "epíteto", that means epithet, for the "Hawk Eyes", and the three words have very different meanings, in the Spanish version is kind the same, so seems that this ideia that someone has given Mihawk a tittle of "World Greatest Swordsman" seems to exist only between the english readers of he manga.
But for some people u cant say anything about bad translations and this things they will not care, because they read the manga just to make powerscales and headcanons