The zero is not assumed, but objective: the state of things before the negotiation.
If before the negotiation, a landslide was already closing on your (uninsured) country house, then after the negotiation the "state of things" is going towards the negative, for reasons unrelated to the negotiation. The question here is about the supposed distinction between that landslide and your opponent's decision algorithm.
If before the negotiation, a landslide was already closing on your (uninsured) country house
I can't find the example this is from.
For a more parable-ic version of this, see here.
Suppose I make a precommitment P to take action X unless you take action Y. Action X is not in my interest: I wouldn't do it if I knew you'd never take action Y. You would want me to not precommit to P.
Is this blackmail? Suppose we've been having a steamy affair together, and I have the letters to prove it. It would be bad for both of these if they were published. Then X={Publish the letters} and Y={You pay me money} is textbook blackmail.
But suppose I own a MacGuffin that you want (I value it at £9). If X={Reject any offer} and Y={You offer more than £10}, is this still blackmail? Formally, it looks the same.
What about if I bought the MacGuffin for £500 and you value it at £1000? This makes no difference to the formal structure of the scenario. Then my behaviour feels utterly reasonable, rather than vicious and blackmail-ly.
What is the meaningful difference between the two scenarios? I can't really formalise it.