This is interesting, but, the "nothing but AI" part is clearly not true and so I'm not exactly sure what was done here.
Why is it not a game? I even played it a bit -- was I hallucinating?
The Twitter thread shows exactly what the AI made. It was used for the JavaScript code and the image assets.
If you don't have a Twitter account you can see the full content on Nitter https://nitter.net/javilopen/status/1719363262179938401 or in my Gist copy: https://gist.github.com/simonw/f7ed52daaa66f849858d17e0d6c1c...