When people say "LLMs don't know they don't know" they're referring to some truism: Fundamentally these people believe LLMs are just pattern matching and intelligence is something beyond that.
So when they see LLMs say "I don't know", their reaction is "it doesn't know what it is talking about", not "wow LLMs actually can tell what it knows!"
Conversely, when they observe a human confidently says something wrong, their reaction is "what a cocky guy" but not "oh so humans are not better than LLMs in this regard."
Because they already believe humans are different and superior, whatever they observe doesn't affect their opinion.
So when they see LLMs say "I don't know", their reaction is "it doesn't know what it is talking about", not "wow LLMs actually can tell what it knows!"
Conversely, when they observe a human confidently says something wrong, their reaction is "what a cocky guy" but not "oh so humans are not better than LLMs in this regard."
Because they already believe humans are different and superior, whatever they observe doesn't affect their opinion.