If I understand, this is an open source project that was in the work for a little while, the game jam part is just the record and replay mechanic. So skirting the rules.
I don't know if the dev of the open source project is also the game jammer however it's a port of Neosparks old beepsaber implementation:
https://github.com/arpruss/OpenSaberPlus
Viewing post in Record Saber jam comments
Yes, this entry is based on OpenSaberPlus. As you said on the livestream (sorry, couldn't watch it live!), I mentioned this on the Discord but I overlooked it while preparing the itchio project page since my submission was already late.
Regarding the rules, thanks for allowing this entry despite stretching the rules both in terms of deadline and code usage. To make things clearer for everyone, I suggest adding a question about the used code sources for the next game jams. Personally, I'm okay if everyone used existing projects like me. As a player, I'd prefer to play a complete game that uses another project as a foundation rather than play an incomplete from-the-scratch game.
I'm not the dev of OpenSaberPlus. I submitted a PR to it last month but got no response - the project seems inactive. Therefore, I had my own private fork with several bugfixes and minor improvements. I have just released the source code of Record Saber, which incorporates all of these. Also, I updated many other aspects of OpenSaber for this project, e.g., new saber sounds, models and improved timing. Almost all of the UI underwent complete transformation for better UX.