People value intelligence and will apply that label to themselves even if it may not be true. It's a hard thing to measure though IQ does it reasonably well. To those who have a high IQ: what is it like? Can you pick up any book, read it, and understand the gist with minimal repetition? Can you infer solid and accurate conclusions based on a small amount of evidence? Is any subject or discipline up for grabs or do you have to have a keen interest in a particular field in order to flourish? What is something you are able to do that you know is because of your intelligence -- the proverbial 1,000 pound deadlift of the brain, if you will.
There is no point to these questions other than curiosity.
There may be no (direct) common reference between the two people, but there is a common reference. If a person hears a sound, and then later their mind reproduces that sound (say, remembering a song) they'd say they 'hear' the sound in their head (or at least I've never met anyone that doesn't unless they're making a fine distinction between the subjective experiences of hearing with the ears vs 'hearing' with the mind.) If someone says 'I don't hear my own thoughts/memories', particularly in reference to a common experience (take the song example from earlier) we can reasonably infer that their subjective experience is very different from someone who does 'hear' their own thoughts.
I know that personally, I am quite capable of hearing (or seeing) thoughts/memories, and the experience is near enough to the real thing that I wouldn't use any other term other than to clarify that I'm not undergoing some sort of self-induced hallucination.