I found this: "Creepage distance means the shortest distance along the surface of a solid insulating material between two conductive parts."
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I believe it is just to describe specific situation. I would not apply Creepage values for situation when tracks are inside PCB - but it doesn't mean that the distance between tracks inside PCB routed on top of each other should be ignored. Even for the material between the tracks, there will be voltage what will cause problems - you just may need to find the safe distance number for this specific material.
PS: I remember these high voltage break through tests from university - we were measuring break voltage of different materials. That was fun