You probably missed the part where they say how many sourceforge projects have only one developer. That clearly says something about the development method, and how often are these promises realized.
I don't think that proves anything at all. If you said to a free software advocate: 'Freedom is pointless because most of us don't exercise most of our freedoms' that would clearly be a nonsense argument. Enumerating open source projects and counting their contributors is barely more coherent than that.