There is some nuance to this. Adding comments to the stated goal "Everyone who interacts with Debian source code (1) should be able to do so (2) entirely in git:

(1) should be able does not imply must, people are free to continue to use whatever tools they see fit

(2) Most of Debian work is of course already git-based, via Salsa [1], Debian's self-hosted GitLab instance. This is more about what is stored in git, how it relates to a source package (= what .debs are built from). For example, currently most Debian git repositories base their work in "pristine-tar" branches built from upstream tarball releases, rather than using upstream branches directly.

[1]: https://salsa.debian.org