Does anyone know what are the artifacts at the bottom left corner of the video? Is it just an issue with the video capture or is he doing something clever like storing data in these pixels?

To save 3 bytes I had to use the address in DI after drawing the maze (just below the last tile row), it contains the positions of the player and ghosts.

You say "save 3 bytes", we say "visualizing dynamic game state, for debugging and educational purposes, and highlighting how small the dynamic state is compared to the static art asserts"

Reminds me of CRAM dots on the Sega Genesis: https://segaretro.org/Sega_Mega_Drive/Palettes_and_CRAM#CRAM...

Storing data in pixels, so it only has one memory segment and can write direct to memory to do the graphics.