He means well, but I think he wrote the piece without thinking it through as deeply as he might.
And prostitution, as you say, seems to occur in most times and places. I think what he meant to say was that, if you write a character who's a prostitute, make her a real, fully-developed person, and not a lazy cliche. Because yes, viewing prostitution as a no go area elides another huge area of human -- and especially female -- experience.