One of their employees says this is in error[2] so hopefully it will be fixed.
I guess signing in with email is pretty much equivalent to contacting Facebook if this is possible to do.
Besides that there are physical retailers that send data to Facebook even though I don't recall giving them any idea identifying info. I feel powerless since I rely on Messenger for communication with friends, who I've tried and failed to convince to switch elsewhere.
[1] https://www.plex.tv/about/privacy-legal/privacy-preferences/
[2] https://forums.plex.tv/t/why-is-plex-sharing-my-activities-w...