So it's facially subjective, but not practically once you include resolving a dispute in court.
I'd even argue that Microsoft may have taken advantage of the board's cult-like blindspots and believes that a court-acceptable qualifying AGI isn't a real enough possibility to jeopardize their contract at all.
"an autonomous system that surpasses human capabilities in the majority of economically valuable tasks."
That doesn't sound too subjective to me.
I think the closest way you could truly measure that is to point at industries using it and proving the theory in the market. But by then it’s far too late.
Having some billions of dollars of profits hanging over this issue is a good test of value. If the "is AGI" side can use their AI to help their lawyers defeat the much better/numerous army of lawyers of a billion-dollar corporation, and they succeed, then we're really talking about AGI now.
Were they expecting it to never happen and were just ready to throw a mountain of lawyers at any claim?