I think you hit the nail on the head. I'll just add for clarity that (unless something has changed) Lemmy can only determine cross-posts based on matching links (whether it's an article, image, etc.). There is no way to correlate text cross-posts, which is why the UI (both the Lemmy web UI and Thunder) put the little "cross-posted from..." note in the body. At the very least, that helps the user find the original post from a cross-post, considering there is no special relationship in the back-end between cross-posted text posts.

Again, this was my understanding of how things worked last time I looked into this.