Amex might very well take the OP's side, and they'd be right to. Credit card companies typically want to protect their customers and are predisposed to help them in matters like this. However, they have absolutely no control over what Meta does after that. They can't force Meta to keep doing business with the OP, even if the reason for terminating business is "I want to punish OP for taking 40k from my account that I didn't think was there."

Editing to add: One avenue that would probably work fairly well would be to get a lawyer to send a mean letter to Meta's general counsel. $40k is enough money to make the hourly rate worth it, and an attorney will be happy to help further if Meta tries to be punitive.