Big Mom used it against Page One because she was actually being serious in that situation.
She's clearly just toying with Law and Kid right now. Even going as far as laughing at the prospect of him "dying" without her even having to do anything.
There is two different personalities to Big Mom. One where she is serious, and one where she is just enjoying herself and being child like.
I swear, she's probably top 5 most misunderstood characters in all of fiction.