Many filterlists (such as EasyList) use "cosmedic filters" which hide parts of websites. Many of these filters are applied to all websites. This Proof of Concept downloads the list of those filters, creates elements which will be hidden by them, and then sees if those elements were hidden.
This is not an "exploit" or an original idea, it is just a more automated type of what many real websites already do. This is also **not** a test to see if your content blocker works!