Okay, I was wrong. I forgot that you have to put the Chromatic into dev mode. The Codex had a card with a unique key on it that you’d input into the MR Updater!
So the magic is the cart, the firmware update and the activation key which unlocks the flashing.
Reddit users have already hacked the MR updater to bypass the key to flash games in their device.
If you want to play it safe, just transfer your roms to a flash cart. 😅
For those wondering, there’s nothing magical about the Codex Chromatic. The cart plays any Chromatic. I’ll try to flash a game on this green one though. Should work in theory.