First, I would like to challenge the assumption that Debian primarily uses SVN f...

First, I would like to challenge the assumption that Debian primarily uses SVN for packaging. We can get a rough idea from the use of Vcs-$name package metadata in the testing distribution:

  </var/lib/apt/lists/*_debian_dists_testing_main_source_Sources.lz4 unlz4 |
    sed -n '/^Vcs-Browser:/n;s/^Vcs-\([A-Za-z]*\): .*/\1/p' |
    sort |
    uniq -c

On my system this gives:

      4 Arch
     72 Bzr
      5 Cvs
      5 Darcs
  24168 Git
     21 Hg
     21 Mtn
    603 Svn

Which shows that git is the overwhelming favorite and about 40 times more popular than SVN for packages which provide VCS metadata.

The sources can be pulled directly from Debian as tarballs using [syncpackage]. Ubuntu maintainers are free to use their preferred VCS for maintaining Ubuntu-specific packaging. Using the same VCS as the Debian maintainer (or upstream developer) is often convenient, but not required.

[syncpackage]: https://manpages.ubuntu.com/manpages/precise/man1/syncpackag...