I know that. You've misread my question. The idea is that the backer won't upload the PDF on file sharing websites because the author will find it and trace it

I know that. You've misread my question. The idea is that the backer won't upload the PDF on file sharing websites because the author will find it and trace it back to the backer using the watermark. But if the backer was anonymous by means of fake information provided to the author at the time of purchase then it's not deterring anything.