Generally the source of a quote is valuable context, and a heavily-referenced topic like this is probably kind of an edge case.
I’m not super-familiar with the backend, but I imagine the composer preview just takes the username from the quote shortcode, while the work of generating the rest (title, link, avatar) is saved until the post is published and “baked.”
Though if we just paste a URL to another topic, the preview does render everything – so it must not be a ton of overhead to do that.