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.