That study only looks at the deterrent effect on existing criminals, not the general populace. I think there is confusion about the word "deterrent". I always thought about 'deterrent' in this context as: "Prisons are a deterrent.. just the thought of serving time in jail will deter anyone from committing a crime." Whereas the study you linked reports: "We have found that criminals who serve longer sentence are no more deterred from committing another crime than those who served short sentences."