Any reason they didn't use the debian apt source repositories instead of git? I tend to try and use deb-src whenever possible, to stay as close to Debian truth as possible.
They cloned the Debian git repo, https://salsa.debian.org/multimedia-team/fdk-aac , and used apt source directly for pipewire. Not sure why the split approach, but it does look like they are relying on a Debian source of truth.
The split approach is because the free version of the library is not yet in Debian. There’s probably some way to pull it from the new queue directly but cloning the git repo is easier.
If you run "apt source" for the package you'll see it - like lots of other packages - point you to the git repository when the packaging is maintained in a public git repository. The Vcs-Tag field in the package lists is the magic that triggers that message.