Your source isn’t as clear on that matter as you claim:
> Police still make over 1 million drug possession arrests each year, many of which lead to prison sentences. Drug arrests continue to give residents of over-policed communities criminal records, hurting their employment prospects and increasing the likelihood of longer sentences for any future offenses.
> Nevertheless, 4 out of 5 people in prison or jail are locked up for something other than a drug offense — either a more serious offense or an even less serious one. To end mass incarceration, we will have to change how our society and our justice system responds to crimes more serious than drug possession. We must also stop incarcerating people for behaviors that are even more benign.
My interpretation is that releasing drug offenders alone will not reduce the prison population significantly. However—and this is a big however—harsh punishments for drug offenders and crimes related to drug use is a driver of mass incarceration.
It was my understanding that legalizing drugs is more about treating drug addicts humanly and offering treatments over punishments. To offer drug users safe usage sites and other benefits so that they are not forced into commit crimes for acting on their addiction. The legalization is only a part of a much broader policy change which may ultimately reduce criminal behavior in addicts, criminal behavior which includes, but is not limited to, possession.
Your source seems to agree with my understanding:
> For example, the data makes it clear that ending the war on drugs will not alone end mass incarceration, though the federal government and some states have taken an important step by reducing the number of people incarcerated for drug offenses.
> Are state officials and prosecutors willing to rethink not just long sentences for drug offenses, but the reflexive, simplistic policymaking that has served to increase incarceration for violent offenses as well?
TLDR; Your source is very much for legalizing drugs, however it acknowledges that legalization by it self is not sufficient in ending mass incarceration.