I don’t understand how a slingshot maneuver off of a central black hole would work. My understanding was that a slingshot never slows you down in the frame of the object you are slingshotting around, it only changes your direction. Since the central black hole is presumably stationary with respect to the rest of the galaxy, this wouldn’t help you in slowing down. Slingshotting around an intermediate mass black hole (if such things exist) out in the galactic disc seems like it would be more useful.
Or maybe there is something about general relativity that changes things?
This is exactly right! It’s a poor analogy for the Cold War both because the total payoff for defection was higher than the total payoff for cooperation, and because the reward was fungible. The cooperative solution is for one side to “nuke”, in order to maximize the total donation to both organizations, and then to use additional donations to even out the imbalance if necessary. That’s exactly what happened, and I’m glad the “nuking” framing didn’t prevent EAs from seeing what was really happening and going for the optimal solution.