Yea, I saw his ted talk a while back. Looking at it again, it seems he's sort of confirmed my suspicion. It sounds like he's aware he's hearing color, rather than some other new perception. And he actually commented on how he conflated sound/color (that is, regular sounds have color). Which tells me it's not actually a new sense, but an old sense used in a new way.
It's really baffling. I'm wondering how intuitive it is for him to just be able to look at something and clearly say what sound it is. Or to hear something and go "yea, that's yellow/orange/red/violet."
In the talk he really only went over how "color is sound to me, lol" and just did a few examples of how he interacts with things. Not so much on the sensory side itself.