The fact that one of the words is a subset of another depending on the context doesn't invalidate the fact that the words are still synonyms when they are used in a specific topic.
Moreover, by what you said before:
> "Exchange" is the critical word. A single promise is extraordinarily different...
I would conclude that you are actually agreed on how I used it in my original comment:
> "and contracts might be dissolved if both sides of the contract agreed. This being an open promise is not a contract but a reaffirmation of a goal"
