I’m talking about the end-user securely downloading packages from the repo, not how the package maintainer obtains the software upstream.

How a package maintainer obtains the software from the source is dynamic and depends on the package. Ideally those releases are signed by the developer. In any case, if the package is poisoned when grabbing the source, it’s much easier for the community to detect than a targeted MITM attack on a client obtaining it from the repo.

I can say that I do maintain a software project that’s in the repo, and we do sign it with our PGP release key. Our Debian package maintainer does verify its authenticity by checking the release’s signature. So the authenticity is checked both at the source and when downloading the package.