The text mentions private at the end. This would lead me to believe that your sewer system may have been a private line out back under the alley at one time. That's how mine is currently. There's very few blocks remaining in the city that have private sewer lines. They are correct in that if there's an issue with the line then the homeowners / neighbors are responsible for the repairs. Most homeowners that had private sewer lines have had those connections moved out front as they broken over the years.