I think we don't disagree about whether elite common sense should defer to cryptography experts (I assume this is what Bruce Schneier is a stand-in for). Simplifying a bit, we are disagreeing about the much more subtle question of whether, given that elite common sense should defer to cryptography experts, in a situation where the current views of cryptographers are unknown, elite common sense recommends adopting the current views of cryptographers. I say elite common sense recommends adopting their views if you know them, but going with what e.g. the upper crust of Ivy League graduates would say if they had access to your information if you don't know about the opinions of cryptographers. I also suspect elite common sense recommends finding out about the opinions of elite cryptographers if you can. But Wei Dai's example was one in which you didn't know and maybe couldn't find out, so that's why I said what I said. Frankly, I'm pretty flummoxed about why you think this is the "No True Scotsman" fallacy. I feel that one of us is probably misunderstanding the other on a basic level.

A possible confusion here is that I doubt the cryptographers have very different epistemic standards as opposed to substantive knowledge and experience about cryptography and tools for thinking about it.

I certainly don't get the impression that one can grind well-specified rules to get to the answer about polling the upper 10% of Ivy League graduates in this case.

I agree with this, and tried to make this clear in my discussion. I went with a rough guess that would work for a decent chunk of the audience rather than only saying something very abstract. It's subtle, but I think reasonable epistemic frameworks are subtle if you want them to have much generality.