Goodhart's law does not cleanly apply here, because the group cares about more than making money, and would bear all the costs of not doing (what observers regard as) being in their interest -- both in that case, and whether potential counterparties regard it as being predictable enough to make reliable long-term agreements with.

To illustrate with an example, your point is like saying that if we had a prediction market for "Will the United States cede Texas to Mexico in 2026?", then the US government would give up Texas just to get that sweet sweet prediction market payoff.

I would agree with a smaller point, that an org would accept minor tweaks it doesn't care about in order to game a market, but this just means it can tolerate being unpredictable about lower-order bits of its decisions. You see that in cases like Trevor Noah making a minor change to a speech to influence a particular bet.