If lots of subjects were using CDT or EDT, they would all be choosing ice cream independently of their soda, and we wouldn't see that correlation (except maybe by coincidence). So it doesn't have to be stated in the problem that other subjects aren't using evidential reasoning--it can be seen plainly from the axioms! To assume that they are reasoning as you are is to assume a contradiction.

If lots of subjects were using CDT or EDT, they would be choosing ice cream independently of their soda iff the soda has no influence on whether they argue according to CDT or EDT. It is no logical contradiction to say that the sodas might affect which decision theoretic intuitions a subject is going to have. As long as we don't specify what this subconscious desire for ice cream exactly means, it is thinkable that the sodas imperceptibly affect our decision algorithm. In such a case, most of the V-I people (the fraction originating from V-S) would be attracted to causal reasoning, whereas most of the Ch-I people (the fraction originating from Ch-S) would find the evidential approach compelling. One can say now that the sodas "obviously" do not affect one's decision theory, but this clearly had to be pointed out when introducing a "subconscious desire."
I agree that once it is specified that we are the only agents using decision theory, screening off applies. But the game is defined in a way that we are subjects of a study where all the subjects are rewarded with money:

(an excerpt of the definition in Yudkowsky (2010))

It so happens that all participants in the study who test the Chocolate Soda are rewarded with a million dollars after the study is over, while participants in the study who test the Vanilla Soda receive nothing. But subjects who actually eat vanilla ice cream receive an additional thousand dollars, while subjects who actually eat chocolate ice cream receive no additional payment.

After reading this, it is not a priori clear to me that I would be the only subject who knows about the money at stake. To the contrary, as one of many subjects I assume that I know as much as other subjects know about the setting. Once other subjects know about the money they probably also think about whether choosing Ch-I or V-I produces the better outcome. It seems to me that all the agents base their decision on some sort of intuition about which would be the correct decisional algorithm.

To sum up, I tend to assume that other agents play a decision theoretic game as well and that the soda might affect their decision theoretic intuitions. Even if we assigned a low prior to the event that the sodas affect the subject's decision algorithms, the derived reasoning would not be invalid but it's power would shrink in proportion to the prior. Finally, it is definetly not a contradictory statement to say that the soda affects how the subject's decide and that the subject's use CDT or EDT.