This is the wrong way to read the paper. The author isn't making a case against decriminalizing drugs. He's saying that even if you decriminalize drugs, you will still need other stronger interventions to work against mass incarceration and, for that matter, the racial disparities in incarceration.
The specific argument is made that if you decriminalize marijuana, it won't make a difference because there aren't that many incarcerations for marijuana specifically.
