They are overlays. If they were tiles, think of how many tiles there would have to be for all the different tiles and terrain they would be on. Also creep edges work on Installation tileset, which creep has been removed on.
ADDITION:
Found it.
QUOTE
tileset\ashworld.grp
tileset\badlands.grp
tileset\desert.grp
tileset\ice.grp
tileset\install.grp
tileset\jungle.grp
tileset\platform.grp
tileset\twilight.grp
I was wrong about the installation thing. It looks like every tileset has their own set.
I am surprised no one bothered wondering what is in the tileset's grp files..