Wasn't the vulnerability triggered by a malicious script that was added silently to the tarball? Reproducible builds would have shown that the tarball is not the exact output of the build. Even though the malicious payload was already in the code, the trigger was not and was hidden
>Reproducible builds would have shown that the tarball is not the exact output of the build
That is not what reproducible builds do. Reproducible builds shows that the compiled binary comes from the inputs. You have to use the same inputs as the distro else it will most likely not match. The vulnerability is part of the input which means that anyone else reproducing the build would have a byte exact copy of the vulnerable library and no discrepancy would be found. Reproducible builds would monitor for when the builds don't match.
In this scenario you could compare release tarbells against the git repository, but that has nothing to do with reproducible builds.