Built on meaning the technology is using Git under the hood, not that it is developed using git.

Edit:

Breaking down the “word salad”:

> Radicle is a peer-to-peer code collaboration platform (“forge”) built on Git.

Peer-to-peer: it functions with individual nodes on the network spreading state for tracking it without relying on a single entity or centralised service.

Code collaboration platform (forge): you use it not just to store code but provides a way to keep track of “patches” (their term for PRs) and issues, amongst other things, to enable multiple people to collaborate on a code base

Built on git: the technology runs on top of git insofar as not only is the VCS just git, but the issues, patches, etc are stored in git. So the project isn’t merely developed using git, but when running the tool yourself it’s still backing everything under git.