conversely, it's reductive to compare it to a generic ban on saying "I don't know"

my first job was a startup I built, starting from being a waiter. There's a lot of people who don't even try, sort of reject the premise of engaging with the question.

Saying "I don't know", then looking at the interviewer, is about the least valuable interaction you can have with an interviewer, and there's a shocking # of people who do that: whether it be freezing up, some sort of implicit commentary on the question, or a form of performance art, whatever the implicit intent is, in 99% of cases, it doesn't serve you at all.

If I ask you how to fluffle nuffle the snorbknobs, you can at least come back with "I don't know, I may have misheard you: I've never heard of fluffle nuffle. Can you give me a hint?". If you're at the end of your rope with tech interviews: "Excuse me, those aren't sensical words, I'm worried you're having a stroke"

"I don't know" and silence is NPC stuff.