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) 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