How tor exit nodes are spoofed

Here's a video in which police traced child pornography image back to his IP address(Tor exit relay)! And he(the one who runs tor exit relay) says that he runs tor exit relay so he has nothing to do with it! https://youtu.be/8PO6qQHygFo

So I want to know how police can trace the child pornography image if tor is used to browse anonymously? Does that mean everything on tor exit relay can be traced or found what?

I think it's because exit tor exit relay is unencrypted! And if I am correct how to encrypt it!

Best regards!