The problem here is that a contract is defined as a document that provides offer, acceptance, and consideration.
Ah, but it isn't.
In order to have a contract you MUST HAVE offer, consideration, and acceptance. Montana had all these things, therefore the compact with the United States is a valid contract.
If A then B.
B.
Therefore A.
Affirming the consequent. All contracts must have offer, consideration and acceptance, but that does not necessarily mean that all things with offer, consideration and accpetance are contracts.