Meeting dishonest ex-supervisors at large international conference -- Should one speak out?
There is a lot going on here. If you want to play power politics make sure that you have the power first. 

You contemplate making an accusation. Depending on local law and custom this could open you to a charge of slander that you would have to defend. Far in the past a duel on the green would be seen as the appropriate response. Now it is lawyers instead of pistols, of course. 

If you can make the claim fairly and can also do so anonymously you protect yourself. If the journalists you intend to speak with can really protect "their sources" (i.e. your identity) it might be possible. But they will need evidence even though they are less susceptible to a charge of slander. 

But an emotional response won't help anyone and might reflect badly on yourself. 

But I see your dilemma. Others may be at risk here if you don't make some attempt. I think that the conference chairs that I have worked with would listen to your complaint if you can contact them. This won't guarantee anonymity, of course, but in the best case it could put other powerful people in your camp. On the other hand, if the committee is somehow aligned with the PIs in question it would be the worst situation of all. 

One rather scary possibility is that you confront them privately and tell them your complaints and that you don't like to see them recruiting others into the same situation you faced. If you know of any others who already faced the same situation and you can confront the PI as a group it would be even better. Some people have no shame at all and some are driven by forces even they can't control, but you might let them think that withdrawal is a better option than continuing. Scary, though. 

Ethics don't require, in a case like this, that you put yourself at risk to save others.