How in the world do they imagine that introducing sex offenders will reduce violence?
I suspect the thought was that a prison population in which just about everyone was in for assault, offenses involving a knife*, attempted murder or murder would be more violent than one in which the concentration of
a priori violent felons was diluted by including some folks convicted of lewd-and-indecent behavior, child porn possession, and rape (esp. of the non-forcible variety -- underage victim, unconscious victim).
Reading the article, it seems the one data point -- the Doncaster prison -- supported their hypothesis: The prison became "more stable".
*This is the UK we're talking about, having almost completely banned private firearms use, they now fret about "knife-crime" in the same overwrought tones the left in the US uses for "gun-crime".