Winning is about how alternatives you choose between compare. By cooperating against a same-action DefectBot, you are choosing nonexistence over a (D,D), which is not obviously a neutral choice.
This becomes a (relatively) straightforward matter of working out where the (potentially counterfactual—depending what you choose) calculation is being performed to determine exactly what this ‘nonexistence’ means. Since this particular thought experiment doesn’t seem to specify any other broader context I assert that cooperate is clearly the correct option. Any agent which doesn’t cooperate is broken.
Basically, if you ever find yourself in this situation then you don’t matter. It’s your job to play chicken with the universe and not exist so the actual you can win.
This becomes a (relatively) straightforward matter of working out where the (potentially counterfactual—depending what you choose) calculation is being performed to determine exactly what this ‘nonexistence’ means. Since this particular thought experiment doesn’t seem to specify any other broader context I assert that cooperate is clearly the correct option. Any agent which doesn’t cooperate is broken.
Basically, if you ever find yourself in this situation then you don’t matter. It’s your job to play chicken with the universe and not exist so the actual you can win.