You don't promote C to the action node, it is the action node. That's the way the decision problem is specified: do you one-box or two-box? If you don't accept that, then you're talking about a different decision problem. But in Newcomb's problem, the algorithm is trying to decide that. It's not trying to decide which algorithm it should be (or should have been). Having the algorithm pretend - as a means of reaching a decision about C - that it's deciding which algorithm to be is somewhat reminiscent of the idea behind TDT and has nothing to do with CDT as traditionally conceived of, despite the use of causal reasoning.