I’ve noticed it doesn’t seem to work if your chosen Library type is ‘Other Videos’.
You get a prompt to select a scanner, which of course you can’t if you pick the Plex NFO Movie Agent.
I was trying to point it to the same folder that my Jellyfin server looks at for a dedicated music video library and see how it handles those .nfo’s. Since the official Plex apps, unlike most Jellyfin clients, don’t allow for 16:9 thumbnails to show on the home screen or libraries unless you pick that particular library type.
The Android TV client also wrongly shows the backdrop instead of the thumbnails, but that’s another client bug (fine on iOS clients). They really are all a bit of a mess right now. 
Don’t worry too much about that last one: Jellyfin’s web client has the same issue. 
I did try importing the files and .nfo’s in a Movie type library and of course that didn’t work either because of <musicvideo> & </musicvideo>.
Changing it to <movie> + using XBMCnfoMoviesImporter.bundle did show the metadata (but the 16:9 thumbnails were of course displayed in 2:3 poster format), so if the NFO Agent were selectable in ‘Other Videos’ and <musicvideo> were supported, this ‘should’ all work.
