I think it's single in the sense that only a single failure in any of those points would render the entire project a failure, not that there's only one point that can possibly fail.
Contrast, say, a single-engine jet plane with a twin-engine jet plane that can still make it to the airport safely with the remaining engine, should one engine fail mid-flight.
Contrast, say, a single-engine jet plane with a twin-engine jet plane that can still make it to the airport safely with the remaining engine, should one engine fail mid-flight.