Technically, that was Urouge doing narration. So all that means is that Urouge thinks Kaido is the strongest.Oda said "In a 1 vs 1, always bet on Kaido". That makes it pretty obvious that Kaido is the strongest person alive.
How can he be the strongest if, according to that same narration, he's been defeated and captured multiple times?
Anyway, there's plenty of loopholes around that kind of hype. If he's the strongest man alive, then Big Mom will kick his ass. If he's the strongest pirate, then Akainu will wreck him. If he's the strongest human, then you claim the other Yonko are monsters. If he's the strongest creature, then you claim the other Yonko are gods. If he's the strongest entity, then Oda will kill him.