I think this is what actually lies at the heart of the book - that Patti and Robert had a connection, a closeness, that made their love for each other inevitable. It is the reason for the book - to try and explain the basically inexplicable: the magic that happens when two complimentary/congruent minds meet.
They were both on the same mission, to make sense of the world through artistic expression. They "got" each other's artistic vocabulary - the icons, the mysticism, the classical feel - and recognised their own shared rock and roll urges.
Maybe it's something akin to a shared sense of humor - you know when someone can make you laugh without saying anything, because you somehow know they are having exactly the same random thought that you are having? You know if you even so much as catch their eye, you will both fall completely apart, laughing hysterically.
Whenever he said things like that, for a magical space of time, it was (as) if we were the only two people in the world.
was acknowledging that shared universe - a place where she was understood - no longer an "outsider".