I don't think they are talking about getting the artifact at the same time as the source or any such thing. They are saying, the metadata required to reproduce a build is not present in that sdist format, and there's no place to attach it. So, if you got an artifact and separately got the source, you couldn't verify it without another source of information about how to do the build itself in exactly the same way. It goes beyond pure reproducibility as well. Without that metadata to set up your environment, you may see bugs or other differences in your build that are not in the distributed artifact.