What's the difference between this and https://tangled.org? This is the first time I've seen it mentioned, and it sounds like it's trying to a build a similar solution.

Also, why are we basically re-implementing a P2P network for VCS when we already have a really good generic P2P networking system which also has Git support?

https://reticulum.network/manual/git.html

IIRC Radicle is ActivityPub/traditional federation style (servers talk in a mesh) and Tangled is built on ATproto which means it's trying to work with a logical single firehose feed of everything everyone does.