Participants: Females aged 18 years or older, who have sold sex on the street and used heroin and/or crack cocaine at least once a week in the last calendar month.

That study is looking at STREET sex workers only, and wasn't assessing it's claim of what percentage are on drugs but instead assessing treatment for them. Their inclusion criteria was that the person was already using drugs and on the street... so ofc 100% of these workers will be using drugs because that's the criteria. Also, SWers who are on the street are not a common group of SWer (even though they are very visible and stereotypical) since vast majority is done through online means or through popular hangouts like bars etc. If you're advertising on the street, it's because you likely can't get into bars or hotels (banned for soliciting or look obviously not well).

Just seems like a bad study that has an n=11 which is super low.

https://en.m.wikipedia.org/wiki/Street_prostitution

Street SWers are different than other SWers^^^