What proof do you have that my theory is not the case? HCS's Neon64, a NES emulator for the N64, uses rather well-written custom microcode to emulate the majority of the features on the RSP rather than tying up the main processor. Animal Crossing emulates the NES games that are available in the game, so it's not all that far-fetched of an idea to consider that perhaps Nintendo got to that idea first.