The section following then mentions signing the pre-processed source code, which I think is the wrong approach. It makes a difficult situation because of how strongly some people encourage signed source code, yet I think autotools is part of the build process and should run on the build server (and double checked by reproducible builds). If people pre-process the .orig.tar.xz they upload to Debian, this pre-processing won't be covered by reproducible builds because it happens undocumented.
The patch for "reproducible tarballs" is quite involved[0] and has rookie mistakes like "pin a specific container image using `@sha256:...` syntax, but then invoke `apt-get update` and `apt-get install` to install whatever Debian ships at that time".