But presumably you understand that your opponent may call you. The relevant MW definition is "a false threat or claim intended to deter or deceive someone." So in this context, it's a claim you won't really leave if you don't get your way. But the other party may decide to get rid of you anyway based on the bluff. (Thinking about it, it's probably reasonable to call it a bluff but that doesn't mean there aren't consequences if the other party calls you on it.)