The problem is the we couldn’t really tell part. Changes made to mature finished projects should be minimal and readable and understandable by humans.
Also rsync is handling copying binary data, it’s a project that’s super sensitive to hardware faults for example, which means it’s not just enough for the tests to pass.
> finished projects
rsync is not a finished project: it has hundreds of open issues (bugs, feature requests, ...).
"Finished projects" are a mythical thing that rarely exists in reality and even less in actually used software like rsync or the Linux kernel.