In an 1V1, the other person is rarely a Yonkou. Even if you bet kaido and he loses, Kaido's still really f*cking strong and worth betting on.
As for the title, depending on translation I could argue any of them to be stronger than him. Strongest beast: Shanks is human so the title is meaningless.
Strongest man: Big Mom says hello.
Strongest living creature: Meaning animal or beast, so even though humans are technically a part of that group, it's easy to say that it doesn't count them
Strongest anything: The title is just plain wrong. It can be. Don Krieg was called the strongest in the world too, and he got curbstomped by mihawk.