When people shut down and struggle to answer, that is very noticeable. If that happened, I'd be asking about their wellbeing and see if there's alternative ways for us to assess.

My problem is candidates that keep being able to talk in ways that to a non-technical manager would sound as if they plausibly know their stuff, but then struggle to offer up any kind of detail when you dig into specifics, while still being articulate yet not giving me any reasons as their answers remain superficially coherent yet descends into technobabble.

I've had candidates telling me they've gone blank. That's fine - I'll find something else to ask about, dial it back, slowly circle back to the problem from another angle, and if I'm fine with everything else we'll discuss e.g. giving them a problem to solve without me there, outside the interview setting. I've had people similarly go blank in front of a whiteboard. That's fine - I ask them to forget about that, and talk through the problem instead.

The candidates I consider to be unable to code are not the ones who freeze up or go incoherent but can pass some other assessment, but the ones that are "confidently wrong" from a fairly high level, and you drill down until they can't explain a simple if statement, while often still talking with apparent confidence.

Maybe I'm sometimes wrong and they're just too anxious to admit to finding it challenging. But if so, that's a much bigger problem to me than if they're struggling with the interview setting more generally.