In U.S. domestic politics, for example, an informal offer can constitute a binding agreement provided one party gives up something of value in consideration of payment in goods or services. A similar principle applies to inter- national politics: not only are formal agreements often the codiacation of arrangements that states would make regardless of a formal offer, but if private and unwritten discussions are meaningless, then diplomacy itself would be an unnecessary and fruitless exercise.

Nope. The article then goes on to describe his research into exactly how NATO discussed how there was a long history of informal agreements during the cold war. The Cuban Missile Crisis was resolved informally for example.