Having read the article I don’t think there is anything to worry about
The whole thing is a scam just like the fake parking tickets what they are sending people is an Invoice as far as debt collectors go you can tell them to take a run and jump or use better words if you wish,
Unless you have been taken to court and a judgement has been made in there favour they are pissing in the wind
To take hundreds of thousands of people to court just wont happen, what they do is threaten you with what looks like legal stuff hoping you will pay, great work if you can get it, there was a situation a few weeks ago with downloaded software and Pirate Bay you may remember they were hacked and all the company details were released and their website defaced they may have even gone into receivership now they were doing simialr.
If thisoutfit gets too big the powers that be on the interweb will do exactly the same and they will be defaced.
As for your own material its easy to watermark stuff as you know and make it unique just add a copyright notice to everysite you post on
I wouldnt be too concerned