This looks good for a pico-8 image, but calling it 256 colors is a bit of a stretch.
I understand that flipping two 16 colors images requires 8 bits per pixels, or 256 possible values, but with a fixed 16 colors palette, you have 16 choose 2=120 maximum colors, probably less for the actual image.
Visually, it feels like a dark version of Amiga's EBH format, witch was 64 colors max and had the same banding artifacts as this image. (On amiga, non dithered images compressed better, so you would have more pictures per disk while still being "64 colors" this way. Might also be an effect of the scanner+software combo witch was often not great at the time...)