It’s literally just using the P2P nature of bittorrent to get peers’ IP addresses (you can do that too! Just start any torrent and look at the peers tab in your client), then using the legal tools to request data about a person breaking the law using such and such IP address.
If you extend this to everything you can have massive surveillance without cookies, MAC address of device, IP address for each package and you know who is accessing what kind of content on internet. So, why wait whenever someone reads article about class 1 drug just brake his door and arrest him, isn't it ?! I mean fork privacy, democracy, and judicial process, just lets immediately arrest everyone :/
Anyway, in German scenario how can you certify that someone was not hijacking your router?
(As per friends case, they had friend who was sleeping over and connected to their Wifi ...)
Some lawyers are even using sites similar to https://iknowwhatyoudownload.com/
It probably costs less to pay the amount than fighting it in court. I think it's capped at 450 Euro or something? But I know some people who got scot free by merely having a lawyer send a response.