From what I understand (I'm no expert) is it runs on the original Atari 2600 hardware using PlusCart, it's not just using it as a throughput for the display.
I read the article at the time or my original comment, but didn't have time to look into the PlusCart at the time. The (perhaps obvious hint) is that the article mentioned the STM32 (which I haven't used personally as my experience with embedded processors is limited so I didn't immediately recognize it when I read the article). I've since looked up the PlusCart. It seem to have two onboard processors. Obviously, the Atari 2600 cannot run such a game, so it must be running on the processor on the PlusCart and using the Atari 2600 as the I/O device. An interesting novelty in some ways...