I don’t see a single word suggesting that the statement in the chapter title is merely a rumor or something people just say.
The worldwide spread rumor that Kaido is the strongest creature in the world?
Which is a thing mostly because the old legends like Rayleigh, Gaban, and even Sengoku/Garp, aren't active anymore. They just stay in their lanes enjoying the rest of their lives. Garp and Sengoku sometimes do some stuff, but it's mostly to support the actual active marines, whether it is the SWORD agents, or the current admirals.
Kaido and Big MOm, despite being considered old gen pirates, were still active agents in the world by the time the SHs faced them. They were the current ||filler|| Yonkos.
Which is why we never heard this "world's strongest" hearsay from Kaido until post-TS AFTER Whitebeard (said to the world's strongest at the time) was already out of the picture.
