That's not the objection I have, it's that we don't have any idea how the bits are arranged inside the PPU (unless someone's decapped one at some point?) Even the fact that there are 9 bits per color is no more than logical induction based on the range of colors seen in the five PPU types that have distinct RGB palettes.

Treating the PPU palettes as ROMs doesn't make any more sense to me than trying to turn, I dunno, the YM2143 fixed instrument parameters (or any of the other lookup tables used by the YM sound devices) into a ROM.