You have a decent argument for the claim as literally stated here, but I think there's some wrongheaded subtext. To try to highlight it, I'll argue for another claim about the "Complete and Honest Ideological Turing Test" as you've defined it.
Suppose that an advocate of some position would in fact abandon that position if they knew all the evidence or arguments or counterarguments which I might use to argue against it (and observers correctly know this). Then, by your definition, I cannot pass their CHITT - but it's not because I've failed to understand their position, it's because they don't know the things I know.
Suppose that an advocate of some position simply does not have any response which would not make them look irrational to some class of evidence or argument or counterargument which I would use to argue against the position (and observers correctly know this). Then again, by your definition, I cannot pass their CHITT - but again, it's not because I've failed to understand their position, it's because they in fact do not have responses which don't make them look irrational.
The claim these two point toward: as defined, sometimes it is impossible to pass someone's CHITT not because I don't understand their position, but because... well, they're some combination of ignorant and an idiot, and I know where the gaps in their argument are. This is in contrast to the original ITT, which was intended to just check whether I've actually understood somebody else's position.
Making the subtext explicit: it seems like this post is trying to push a worldview in which nobody is ever Just Plain Wrong or Clearly Being An Idiot. And that's not how the real world actually works - most unambiguously, it is sometimes the case that a person would update away from their current position if they knew the evidence/arguments/counterarguments which I would present.
This does necessitate that the experts actually have the ability to tell when an argument is bad.
[Cross-posted from Facebook]
"In the moment when I truly understand my enemy, understand him well enough to defeat him, then in that very moment I also love him. I think it’s impossible to really understand somebody, what they want, what they believe, and not love them the way they love themselves."
- Ender Wiggin (from Ender's Game, by Orson Scott Card)
Maybe the following is obvious, but it wasn't initially obvious to me so I'm sharing it anyway.
The term Ideological Turing Test (ITT) was coined by Bryan Caplan and refers to whether someone can articulate and argue the views of an opposing ideology or belief system so convincingly that others cannot tell the difference between the person's articulation and that of a true believer.
Let's use the term Complete and Honest Ideological Turing Test (CHITT) to refer to an ITT where not only can you "fool" observers, but also:
Claim: It's impossible to pass a CHITT unless you actually agree that from some perspective the other side is rational. Furthermore, you must not have any knockdown arguments for why that other perspective is incorrect. That doesn't mean you need to *agree* with the other perspective, just that you don't have a knockdown argument against it.
Reasoning: By the specified criteria, you need to think that no matter what arguments you yourself would throw against the other side, you would not make the other side look irrational even to expert observers. You can of course still have some set of priors that lead you to your true position, while the other side has different priors that lead to their position. But why do you go with your priors and not theirs? If you have a knockdown argument for why your priors are correct, then that's part of the arguments you could throw at the other side, and we're back to where we were before. So it must be that you don't have any knockdown arguments for why you should go with your priors over theirs.