But then that means at the end of the series, when the narrator calls Zoro the strongest swordsman, there's again no actual reason to believe him then right?
Like even if Zoro beats a guy stronger than mihawk, theres no actual proof that that guy was the strongest, and if Zoro beats a guy even stronger than that, there's again No proof that even that guy was the strongest.
Like even if Zoro beats the strongest swordsman we actually ever see in the series, we know random strong people exist in the world such as Fujitora and greenbull so once again, Some off screen guy could still be stronger than Zoro.
See how that goes?
The series doesn't rely on what the narrator calls it. What if the narrator never calls him that? Are you going to claim he's not the strongest, because the narrator didn't call him that?
The scenario you presented doesn't accomplish Zoro's dream. The core of Zoro's dream is to become the world's strongest swordman that his name is Renowned even in the heavens. You don't become renowned in the heavens by beating somebody without actual proof. So that scenario you're presenting simply can't happen. That means even if there is a swordsman who's power is hidden to the readers so far, they will fight Zoro where people can witness his power.
Fujitora & GreenBull would be bad examples, they were renowned for their strength to an extent even prior to the Marines, hence being called monsters when they entered.
This scenario can't exist, because the entire point of Zoro's character is to be the strongest swordsman of all time. Meaning Oda will make it so that no other character exists other than Zoro who is the strongest swordsman. That's simply what the writing is headed for. And if some off-screen guy were to be called the World's Strongest Swordsman, burden of proof would be on them to prove they are stronger than Zoro. We don't need the narrator to tell us that Zoro is the WSS, when the purpose of his final fight is to make him the WSS, hence accomplishing his dream.
The ultimate antagonists of the series are the Celestial Dragons, and their heads are the Gorosei, individuals who are based on influential politicians of the past, they have physiques of fighters. It would make absolutely no sense for Mihawk to be the final fight for Zoro, unless Mihawk will become a bad guy, the likelihood of that is next to 0. Does it mean ZvM fight won't happen? No, it can still happen, but just before the Final War. You can even have the world recognize Zoro as the World's Greatest Swordsman going into the final fight.. and then pull out the Gorosei man, who is the true strongest swordsman in the world.
WSM/WSC/WSS - are all titles limited to the knowledge of the people of One Piece.