I believe the reasoning behind that design was due to how Plex currently handles multiple editions (they’re handled as separate movies). As such, the toggle would be the easiest to implement - both from a design and functionality standpoint. Plex wouldn’t have to rewrite so much of their movie page logic to implement the toggle switch. It’s a relatively simple addition that doesn’t disrupt how editions are handled.
I think it’s also important not to encumber the average viewer with too many choices before they can play a movie. The movie page is already quite crowded, which is why I think placing the edition selection near the other buttons would be too much. The edition selection should be clearly visible on the movie page, while not being a distraction. Ideally, library admins should be able to mark a certain edition as the default, which is played first if the viewer doesn’t override it manually.