I don't think that being able to state the strongest counterargument to your position is a good proxy for the ideological Turing test (or vice versa). Most people with strong political views tend not to have very strong arguments for those views, precisely because they don't carefully consider counterarguments from the opposing side. So if I were the judge in an ideological Turing test, and the test subject made a sophisticated and nuanced argument for his position, one that is constructed as a strong refutation of the opposing side rather than just as a signal of comradeship to people on the same side, I would be suspicious.
The best way to pass an ideological Turing test is to understand the typical arguments made by the opposing side, and that is very different from understanding the strongest arguments made by the opposing side. So I don't think passing an ideological Turing test is an adequate indicator of the sort of epistemic standard you're trying to get at here. It might work if the attempt is to discriminate between partisans who are explicitly known for being intellectually sophisticated (say, Caplan vs. Krugman), but that is a much more specific sort of Turing test.
I would agree with you if the purity of the test were the goal, but my understanding is that the ideological turing test is a means to an end, that end being both people working together to steelman both sides of the debate. Once it has been established that each person fully understands the strongest version of both sides, a true debate can begin.
So you're playing the credence game, and you’re getting a pretty good sense of which level of confidence to assign to your beliefs. Later, when you’re discussing politics, you wonder how you can calibrate your political beliefs as well (beliefs of the form "policy X will result in outcome Y"). Here there's no easy way to assess whether a belief is true or false, in contrast to the trivia questions in the credence game. Moreover, it’s very easy to become mindkilled by politics. What do you do?
In the credence game, you get direct feedback that allows you to learn about your internal proxies for credence, i.e., emotional and heuristic cues about how much to trust yourself. With political beliefs, however, there is no such feedback. One workaround would be to assign high confidence only to beliefs for which you have read n academic papers on the subject. For example, only assign 90% confidence if you've read ten academic papers.
To account for mindkilling, use a second criterion: assign high confidence only to beliefs for which you are ideologically Turing-capable (i.e., able to pass an ideological Turing test). As a proxy for an actual ideological Turing test, you should be able to accurately restate your opponent’s position, or be able to state the strongest counterargument to your position.
In sum, to calibrate your political beliefs, only assign high confidence to beliefs which satisfy extremely demanding epistemic standards.