They did. The unit tests covered that piece of functionality. The real issue with the story being why didn't any other the other dev's run the unit tests after making the change?
The only thing shitty about it was that it didn't adequately explain the problem it was solving. However, that isn't apparent by looking at the code, the only way we could know that is either by reading/running the tests or asking the original developer. There would be no reason to rewrite until you had a reason.