From a skim, it looks like their main point is that with 8 movie ratings, it's trivial to link two datasets (eg anonymized Netflix rating to public IMDb profile).
I have thoughts like that all the time but then I remember how many people have accounts at xyz at all and how often I created an account on abc, made a list or two and never returned (until I did). I have over 600 logins saved in my password manager and "holy fuck shit" some of the data on these sites is still "part of me", I just don't actively spend time on it. It's all quite usable.
thank you, that was my first thought. there's hyperbole and then there's whatever that claim was. I don't want to call it a lie so lets just call it a gross exaggeration.