Does prison reduce crime? Yes, obviously

He moves the goalposts halfway through, defining "crime" as stuff that happens outside jail.

So rapes, murders, drug dealing etc within jail don't count.

He also seems to acknowledge that it's expensive, which I believe is the steelman version of the argument he thinks he's debunking:

There's better things to spend money on that will prevent the crime happening in the first place.

e.g. if an addict steals to feed his habit, spend money on providing them with free drugs and healthcare to deal with their issues and spend even more money on whatever prevents people falling victim to these problems in the first place.

There's a Ted Talk (appropriately enough) from someone trying to reduce the number of death row prisoners executed. He concludes that preventing the crime with social spending is the cheapest option and totally avoids the "death penalty/prison" works/doesn't work argument.

https://www.ted.com/talks/david_r_dow_lessons_from_death_row...