I've interviewed people who were employed in senior developer positions in fairly large companies who could not write code. I don't mean they wrote bad code, I mean that I could not find a way of promoting them to plexplain even basic programming concepts or write a single line of code.
They're not near the developers you want to hire. But they are better at what should be done than some people who have developer jobs.
Some people freeze during interviews, like complete mental shutdown. And it's a downward spiral, once you start freezing it gets worse and then even simple questions become impossible.
That, or you were dealing with professional liars.
For what it's worth, I probably have given off this vibe in interviews in the past, despite being (IMO) a decent programmer.
My brain just shuts off during some interviews and I can't recall even the simplest of things. I forgot the name for ternaries in one of my interviews, despite these being things I use more or less daily.
I'm not sure what it is really. I do fine in exams/tests. I don't have any kind of anxiety on the job or otherwise. I don't even really feel anxious during interviews either, but my brain just goes poof and I can barely form a coherent sentence anymore out of the blue, never understood it.
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.
Measuring soft skills and knowing how to hire the people who have them when your organization needs them is a different problem imo. Obviously there are problems with giving guys with decades of experience graph theory problems when the problems you want to solve are more abstract and organizational.
We're not talking complex problems here, because I firmly believe those kinds of problems do not belong in interviews as coding problems. Maybe spoken about as higher level problem solving. We're talking checking very basic stuff and stressing to them I couldn't care less about whether the syntax is correct. Usually we'd work our way "down" to those kinds of problems once I started getting suspicious about their abilities based on higher level answers.
They're not near the developers you want to hire. But they are better at what should be done than some people who have developer jobs.