They fail to mention the reason the prison population soared in the 70's and 80's, because of ultra-harsh prison sentencing for drugs. In retrospect, those laws appear to have been deliberately designed to create a massive and permanent prison population, far beyond what locking people up only for non-consensual crimes could ever sustain.
Now, most of those laws have been rolled back. In the past 10-15 years the number of people locked up at the state level for drug crimes is down 30% even though drug arrests remain high. And those still getting locked up are getting shorter sentences. (though over 40% of inmates at the federal level are still there for drugs)
I'm not sure why they failed to mention such a key issues related to incarceration. They repeatedly refer to the surge in crime in the drug war era as a "crime wave". And they link to 3 other pro-drug war articles by the same author. Maybe Keith Humphreys had a bad trip in his youth and now he's making it everyone's problem.