Truth. Law and Kid realized they couldn't beat her with conventional means, Big Mom's raw power is just undeniable.
I love how Oda has written Kaido and Big Mom.
Kaido is a merciless, methodical beast who beats the shit out of you slowly over time, while testing your strength. He's like a wild animal that fights to test their own power.
Big Mom is an unpredictable, chaotic force of nature who does whatever she pleases, and can change her mind at any given time, like the weather itself.
Both are raw forces of nature: the cunning cruelty of a beast, and the raw dominance of the weather/climate itself.