I think what might be confusing is that your decision depends on what you know about the paperclip maximizer. When I imagine myself in this situation, I imagine wanting to say that I know “nothing”. The trick is, if you want to go a step more formal than going with your gut, you have to say what your model of knowing “nothing” is here.
If you know (with high enough probability), for instance, that there is no constraint either causal or logical between your decision and Clippy’s, and that you will not play an iterated game, and that there are no secondary effects, then I think D is indeed the correct choice.
If you know that you and Clippy are both well-modeled by instances of “rational agents of type X” who have a logical constraint between your decisions so that you will both decide the same thing (with high enough probability), then C is the correct choice. You might have strong reasons to think that almost all agents capable of paperclip maximizing at the level of Clippy fall into this group, so that you choose C.
(And more options than those two.)
The way I’d model knowing nothing in the scenario in my head would be something like the first option, so I’d choose D, but maybe there’s other information you can get that suggests that Clippy will mirror you, so that you should choose C.
It does seem like implied folk-lore that “rational agents cooperate”, and it certainly seems true for humans in most circumstances, or formally in some circumstances where you have knowledge about the other agent. But I don’t think it should be true in principal that “optimization processes of high power will, with high probability, mirror decisions in the one-shot prisoner’s dilemma”; I imagine you’d have to put a lot more conditions on it. I’d be very interested to know otherwise.
I agree it is better if both agents cooperate rather than both defect, and that it is rational to choose (C,C) over (D,D) if you can (as in the TDT example of an agent playing against itself). However, depending on how Clippy is built, you may not have that choice; the counter-factual may be (D,D) or (C,D) [win for Clippy].
I think “Clippy is a rational agent” is the phrase where the details lie. What type of rational agent, and what do you two know about each other? If you ever meet a powerful paperclip maximizer, say “he’s a rational agent like me”, and press C, how surprised would you be if it presses D?