Gentlemen, we need to take a moment to talk about context here and the importance of context clues. We have one figure to observe here.

Whitebeard is called the "World's Strongest Man" by the narration.
Kaidou is "said to be the World's Strongest Creature" by the narration.

Do you see what I see? No? Very well, then let me help you out.

First of all, the narration is just that. Narration. The Narrator is nonspecific to the story as an entity and mostly exists as a narrative tool for reader engagement on the author's part. The narrator does not, nor has ever truly entailed the outcome of the story, merely acting as a contextual and commentary device for the panels you find the narration present on. But the narrator leaves behind clues that, depending on the interpretation, give us further ideas about the setting or the world within these pages.

When the narrator calls Whitebeard the "World's Strongest Man", this is merely flowery text/wording. A description. As a commentary device, this wording is meant to give us an idea of Whitebeard's status and importance, as well as hype up his advent as a Yonko and one of the most successful pirates in history. But what the narration does not do, or rather, does not receive from this is any further backing. In other words, the narration is the sole entity that implies the meaning of this title. We at no times see any character press a similar claim, nor do we have anyone else directly appellate Whitebeard as "World's Strongest Man". A character's own view would be greater than the narration's in this aspect, as signals to us that the narration wasn't bullshitting us at any point. But nobody does this. Whitebeard is venerated but we see called things like "loser" or called old instead.

Nobody, not a single soul in the setting refers to Whitebeard as "The World's Strongest Man". There are no clues given to this context AT ALL.

This is entirely inversed for Kaidou.