context also says its a belief of skull
in the same quote another character cant make out what to think of shanks .
its an opinion not a fact . the title tags however is undeniably a fact not from the viewpoint of one character . if you have read enough of these novels some marines had contrasting viewpoints on the wss title as well.
wb was the strongest pirate touted as the man on par with roger in strength and when roger passed it was crowned his era by shiki another one of roger's rivals mind you.
a title killer referred to kaido as only in current time.