Of course Paul could be wrong, and then you need to calculate probabilities, which is a trivial calculation that does not depend on a chosen decision theory. But the problem statement as is does not specify any of it, only that he is sure that only a psychopath would press the button, so take it as 100% confidence and 100% accuracy, for simplicity. The point does not change: you need a good specification of the problem, and once you have it, the calculation is evaluating probabilities of each world, multiplying by utilities, and declaring the agent that picks the world with the highest EV “rational”.
Of course Paul could be wrong, and then you need to calculate probabilities, which is a trivial calculation that does not depend on a chosen decision theory. But the problem statement as is does not specify any of it, only that he is sure that only a psychopath would press the button, so take it as 100% confidence and 100% accuracy, for simplicity. The point does not change: you need a good specification of the problem, and once you have it, the calculation is evaluating probabilities of each world, multiplying by utilities, and declaring the agent that picks the world with the highest EV “rational”.