They didn't have a better definition of AGI to draw from. The old Turing test proved to not be a particularily good test. So lacking a definition money was used as a proxy. Which to me seems fair. Unless you've got a better definition of AGI that is solid enough to put in a high dollar value contract?
That's true, but the $100 billion requirement is the only hard qualification defined in earlier agreements. The rest of the condition was left to the "reasonable discretion" of the board of OpenAI. (https://archive.is/tMJoG)