I don't really like that argument. Even granting that you should consider the possibility that you are a simulation running in the box (you might believe that this is all but certain), I'm not sure you have reason to let the AI out. Consider:

Case 1: You are a simulation running in the box.

Then your decision whether or not to release the AI has no impact, and whether or not you (and copies) will be tortured is out of your hands.

Case 2: You are the "real" you, outside the box.

Reduces to the same scenario but without remarks after "the AI adds. . . ." This may still not be trivial, but I suspect a cost-benefit calculation might show that unboxing the AI would have consequences worse than the torture of a million boxed copies. (If not, is the box even relevant? -- simply creating the AI unleashes so much evil on the world that it doesn't matter whether you unbox it.)

(Is there a refinement of the scenario where you can be a simulation but still believe your choice has an impact on your punishment? Probably. For example each copy could get 500 years of torture for its own choice, plus 500 years if the real you does not unbox the AI. This refinement would force us to deal more directly with the AI's threat.)