If you want some statistics and such...I've got nothing. Like most of us, I do have an opinion though.
My initial reaction would be that the Myth listed was instead a Fact. Given that someone has it otherwise, I try to step back and figure why that would be so.
Legalization or decriminalization would reduce a barrier to entry into the business. As such, there would be more available women for pimps to prey upon. But given that it is my opinion that the main barrier to entry relates to stigma, rather than legality, I'm not sure Legal or DeCrim would increase the entry of women much.
Legalization or Decriminalization would most certainly come with some degree of regulation. Health certification, registration, licenseing, etc...or some combination of all that...would probably make more known the identity of the gals that participate. As such, pimps could have better access to prey upon them. But I would think the ability of a gal to ply her trade more openly, without the need for a sponser, would more than offset an increase there.
About the only place I can see that pimp and trafficking would increase would be at the bottom end of the spectrum. I get the impression that those people are very persuasive to young, poor and uneducated gals. Leg and DeCrim would remove one argument from that young gal to say no...and allow a persuasive pimp to persuade.
But at the middle or upper end of the spectrum, I don't see that such would increase pimping or trafficking...and can see that it might slightly reduce it. But in a vaccum...legalization & decriminalization...would, IMHO have very little effect on things (other than maybe a long term criminal record of the lady) until there was a significant move in the stigma attached to the trade. I'm sure none of the gals want soliciting on their records...but my belief is that they don't want it related to the stigma attached to such an arrest or conviction...not the arrest or conviction itself. No one wants a Public Intox arrest either...but if you get one, particularly when you are young, who really cares.