Yes, there’s been several regressions that would’ve been caught by the original tests, but missed by the new vibe-coded tests.

That is directly contradicted by what the developer of rsync wrote in the linked article:

yes, there were regressions in some use cases of rsync in the 3.4.3 release. ... None of those cases were covered by the existing rsync test suite or by all the manual testing I did (yes, I use rsync, I don’t just develop it).

It's possible that somebody in the issue you linked to pointed to a test that would have caught one of the regressions, but I was not able to find it in the 327 comment mess. A direct link would be appreciated, if that is the case.

But I doubt that you will find such a comment. Because I tried running the 3.4.1 test-suite with the 3.4.3 binary, and all tests passed