No it's not. A character being the one deliver information gives reasonable doubt, meaning you can go "This information might not be true." But you still need evidence that it's not true to back that up BEYOND a reasonable doubt. Otherwise, I can go "Pluton doesn't exist. Oh, a character said Pluton exists? They also said Skypeia doesn't exist, so things a character says are wrong. Oh, Green Bull was said to be an Admiral? But this guy said Skypeia doesn't exist, so things a character says are false information."
The fact that Oda has had multiple characters and the narrator cite Kaido as the strongest creature, and even himself reinforced this when he made the joke of "Mothers are the strongest in the world, even stronger than Kaido(the fact that he chose 'even stronger than Kaido' to emphasize the joke)," are signs that, right now, the audience is meant to think of Kaido that way. Even though the chance of this information being retconned exists, it's still what's presented to us as fact right now.