That's wrong. The Super Gameboy worked exactly like this.
It was a SNES ROM which displayed an external framebuffer using PPU sprites. The external frames came from a hardware Gameboy CPU, but could have just as easily come from anything else - an Atari 2600, an XBox One, an iPhone, etc.
It was a SNES ROM which displayed an external framebuffer using PPU sprites. The external frames came from a hardware Gameboy CPU, but could have just as easily come from anything else - an Atari 2600, an XBox One, an iPhone, etc.