At this point in the movie, we know that eccentric millionaire Arcane wants three things: the girl, the notebook and the creature. His current high score is one out of three, and she’s probably not thrilled about being called “the girl” as often as she has, so far.
Agent Alice Cable is currently involved in a high-stakes game of keep-away involving the notebook, which is full of important secrets. The notebook is now in the care of the creature, who should be but is not currently destroying it by chucking it into the swamp water. I mean, if it’s vital for the world that Arcane doesn’t get his hands on the notebook — and I am not entirely convinced that it is — then why don’t they tear it up, dunk it in the water, and let the tannic acid take it from there?