I see. No, I understood it the way it is, as introducing it as a new hard dependency in git 3. I suppose it is a pilot for making it mandatory for contributions / incrementally replacing the existing code in the future, though.
Git is pretty modular, and it already includes multiple languages. I guess that significant parts of it will remain in C for a long time, including incremental improvements to those parts. Though it wouldn't surprise me if some parts of git did become all-Rust over time.
