I’ve become rather numb to numbers lately. $6 million is a tremendous amount of money to most people, especially for a guitar or a house, depending on the location.
The guitar does have a very specific history that is well documented which must help. I think you are right about it being more about Kurt Cobain and Nirvana and it being the guitar he played on the only decent sounding MTV Unplugged album ever released, which in itself is pretty famous.
I don’t know how often famous guitars played by famous guitarists are made available or how much people are willing to pay for them.