I think you’re just expecting a bit much integration between the Plex app and the Fire TV Interface here. It’s just a simple trigger/matching thing. (“If ‘Plex’ app highlighted = display content from advertising partner ‘Plex’”). The status of the app or its configuration has nothing to do with it. The deal is between Amazon and Plex for ad space.
Note: You could sideload Kodi onto the Fire Stick and then install the Plex for Kodi plugin on it (or that fork of it now, I guess) and that would allow access to the Plex content without any promotional posters on the home screen.
