I think, and as this post suggests, it's much more the case that "Kurt Cobain had very good instincts for someone completely untutored" which is a different thing altogether.
I'm remembering a scene in Hampton Hawes's autobiography where a well known piano teacher was telling him his students were starting to ask how to play like Hampton. He tells him he wants to give him lessons to help his technique, which he thinks will help his natural talent even more, but Hampton finds it boring and never goes back. The teacher framed Hampton's check he used to pay for the lesson and put it on his wall. All that's to say having great ears can bring you a long way.
But yeah, feel, instinct, and having good ears can carry you a long way especially for solo artists. I'm still glad I can read music and wouldn't trade that for (almost) anything, though.
But also... what's John Lee Hooker's secret of the guitar??? Don't hold out on us!
But the gist of that method actually sounds a lot like how my music teacher approaches things, he won't let you move on in your improvisations on a new tune until you can get a feeling from playing just half and whole notes on the changes. His teacher was Lennie Tristano so he's very driven by developing that ear instinct.
I am very clear though, that "genius" and "intelligence" are unrelated but sometimes coincident.