In closing, legal prostitution might increase sex trafficking, but your source isn't the proof of this
Can I add on to this? I work for a Cambodian counter-trafficking in persons organization. Cambodia is a country where there was a law against trafficking in persons prior to 2008, but it wasn't until then that there was a serious law on the suppression of human trafficking. Since 2008 the number of brothels in the country has closed, and so it's easy to see that there are less people physically locked in a room and being forced to have sex. At the same time, there is still an enormous population of bars, karaoke parlors, clubs, beer gardens, hotels, massage parlours, and homes where you can buy sexual services. If there's a continuum from "Sex for the sake of enjoyment" at one end, and "Physical coercion" at the other, there's a lot of people who have ended up somewhere in the middle in Cambodia. There's a lot of women (and some men, and some transgender folk) who are engaged in sex-work because they have no other option, and are emotionally, financially, or religiously coerced to a smaller or greater degree.
I think a decriminalization law would probably have the same effect, though I could be wrong. You would probably see less people who were clearly "pimped" in the worst sense of the word, but more people who were taken advantage of legally and within the rules. It would really interest me if someone would do an in-depth study on camgirls and see to what extent there is bullying, manipulation, and coercion towards that population. Someone get on that.
and there's also evidence that the 'epidemic' of sex trafficking isn't as extreme as you may be led to believe.
I tend to agree with this. Though there are certain communities or areas where sex-trafficking (or more often Commercial Sexual Exploitation) is rampantly happening around South-East Asia, there are other forms of trafficking that are much larger (fishing and construction, in particular, for men, Domestic work for women, bride trafficking to China, and sexual exploitation within communities to name a few) the "advertisement" for sex-trafficking seems to dominate the field.