Discourse decided to do its thread context in a way that's currently incompatible with projects like Mbin or Lemmy. Those expect threads to be represented by some kind of post (Page, Article, Image, etc objects), while Discourse decided to use an OrderedCollection, with the first item being the opening post.
Even if Lemmy decided to add support for OrderedCollection threads, there's another issue though: the barrier for initial federation is too high. Discourse only gives you the fediverse handle of its categories, but to federate them in, you need their JSON-LD URL, not the handle. And Discourse decided to use separate URLs for its HTML and JSON-LD pages, with no way to derive the latter from the former. So to initiate federation with a Discourse category, you have to manually do a WebFInger query to get its URL to then give it to your instance. I think most people probably fail this hurdle, either out of laziness or lack of know-how.
Also, I don't know about Lemmy, but the Mbin instance I'm using seems incapable of processing Discourse categories anyway. Not sure why that is.
That means Lemmy is entirely reliant on Discourse users replying to posts that are visible to Lemmy, similar to Lemmy-Mastodon interaction. But with less users, hence less overall interaction frequency.