That's one way to spin it but the article does not say that nor is that really true.
Or in short: Something being likely does not equal fact.That legalised prostitution increases human trafficking inflows is likely, but cannot be proven with available evidence.
Wait, they are against it despite the likeliness of increased trafficking?The researchers also note that other reasons might speak against prohibiting prostitution despite its impact on human trafficking.
...and that's the most important thing. In order to understand the situation you have to look a bit closer. I can only speak for Germany not for the other countries. Prostitution - sometimes declared as oldest profession of mankind - has been illegal for a long time and you barely got any social or medical support. The move towards legalisation was done out of pragmatism. Naturally as it is some countries end up as portals and transfer sites even if they did not chose to be it. Germany was one of them and actually still is when it comes to international crimes - I am talking mafia crimes. So in order to prosecute the more hard and actually cruel cases the police established good contacts with the prostitution scene which helped to separate the wheat from the chaff. Eventually left-wing and liberal politicians took an interest in legalizing part of it in order to improve living conditions and combat crime more effectively. Crimes like child prostitution, slavery and forced prostitution and various forms of drug-dealing is still illegal and are still prosecuted as high-severity crimes.
The article concludes: “The likely negative consequences of legalised prostitution on a country’s inflows of human trafficking might be seen to support those who argue in favour of banning prostitution, thereby reducing the flows of trafficking. However, such a line of argumentation overlooks potential benefits that the legalisation of prostitution might have on those employed in the industry. Working conditions could be substantially improved for prostitutes—at least those legally employed—if prostitution is legalised. Prohibiting prostitution also raises tricky “freedom of choice” issues concerning both the potential suppliers and clients of prostitution services.”
With that said I don't think banning it again would exactly help it. It would create dark figures where there had been pretty clear pictures by now and regardless what has been said about not signing treaties and agreements and what: It is actually actively prosecuted here and there's been a good deal of prevention going on here. In that even the EU agrees and since Germany did actually sign this convention. Where it lacks though is the level of protection of victims which is mainly due to immigration laws and that has been the actually real issue since years, not the lack of fighting against human trafficking or prevention of it but the lack of protection of those falling victim to that due to being declared "illegal immigrant" regardless whether the affected person wanted it or not. And since that has to be improved first and thus legal processes being improved and sped up first signing it would not help. But it is very likely that this will happen before the next elections this later this year still.
So see the EU as a whole is nowhere near legalizing slavery nor is Germany hell-bent on legalizing human trafficking quite the opposite, they are just slow and if you were German yourself you would not be surprised. Stuff can sometimes take more than half a decade and even up to a decade before fully handled by law. Since Germany did sign the preliminary convention already they couldn't back out anyway even if they truly followed a malicious agenda of neo-slavery. I know this might come over as unbelievable but most Germans don't approve of slavery so it is actual a self-interest to work against it on all levels including protection of victims.