The site seems to be some sort of offshoot from a channel where a game developer used to screencast himself developing a game: https://handmadehero.org/watch
From what I can piece together, Handmade Network periodically has some sort of Jam for a few days where a few people hack on projects following some theme.
The most recent theme was Lisp: https://handmade.network/blogs/p/7390
Flip was built for the Lisp Jam. You can see a demo about it here: https://discord.com/channels/239737791225790464/485261036308... (though you'll need to sign up to that Discord group)
There was a discussion about it, a sort of debrief after the Jam, that you can read at https://discord.com/channels/239737791225790464/707742863076....
It's already sparked a bunch of ideas for me: https://mastodon.social/@akkartik/104780599045128839. So decent payoff on being bored and finding this community :)