It also says that democracies have higher rates of sex traffic than non democracies. Correlation doesn't equal causation.
It also doesn't say that criminalization produces better results overall, pointing out that "While trafficking inflows may be lower where prostitution is criminalized, there may be severe repercussions for those working in the industry. For example, criminalizing prostitution penalizes sex workers rather than the people who earn most of the profits (pimps and traffickers)."
This is also not consistant for every nation which legalizes prostitution.
"[C]riminalizing prostitution makes sex trafficking more likely. One widely recognized consequence of prohibition is the formation of cartels, which in a black market are more likely to use violence. This violence drives some producers out of the market, leading to higher prices and large criminal enterprises with monopoly power. Instead of breaking apart sex-trafficking rings, prohibition increases their profitability, making trafficking more appealing to criminal enterprises...
After legalizing prostitution in 2003, New Zealand found 'no incidence of human trafficking.' Moreover, legalization made it easier for sex workers to report abuse and for police to prosecute sex crimes." Abigail R. Hall-Blanco, PhD, Assistant Professor of Economics at the University of Tampa