Kaido's rumored title is subject to interpretation sure but it reflects what many in the world believe.
Akainu was seen taking WB's head and could've easily been boosted to Wsm by some World Government propaganda,but Oda chose to not go that way.
Mihawk is Wss,if he was Wsm,he would have had the title or even rumors around it Postskip.
The only one we can make a case for is BB who was unknown to the world only 2 years ago and now is still growing,so he can surpass Kaido later or prove he was already stronger and it will make sense.
At the end of the day,whether it is confirmed titles or rumored ones,all of them are interpretations of what people In the One piece world believe,no one can logically know if someone stronger can pop outta nowhere at any point.
If kaido is not the strongest due to it being a rumor,then no one in the world currently has any case to be as they lack World strongest Portrayal or even those rumors.