Threaded is also called nested or tree by some. It’s where the responses are directly grouped under the posts to which they respond rather than in a straight chronological time line where posts are placed by the time they were posted regardless of what they are responding to. It’s what us old guys cut our teeth on (FWIW I’m just shy of 60). Threading started falling out of favor 15-20 years or so.
The expanded reply function in Discourse duplicates posts in a thread when the feature is triggered. For example when I expand the replies under a post I get the replies that were made directly to that post (as opposed to using the reply button at the bottom of the thread). The posts that are displayed in the expanded reply mode are also displayed where they would normally be in the chronological listing. This doesn’t serve the user well because they don’t know if they have read that post until, well, they read or start to read it again. As the feature currently operates I don’t see it as an effective UX design.