I assume it is meant to not only have hashes of the final output, but also securely record all steps done by the right people and the right inputs during building of the software. But this solves the wrong^1 problems while still leaving the door open for malware. Reproducible builds are the right step forward if you actually care about security of free software ecosystem.
1. I guess this depend on your position. In the context of remote attestation, I think this is also even dangerous / evil.