(I'm developing it.) It's inspired by Reddit & HN, and has some additional features that makes it simpler to navigate large discussions, e.g. instantly jumping to the parent post (if it's far away) to refresh one's mind, and then jumping back. I made a video: (scroll down to "Jumping to the parent post, and back") https://www.effectivediscussions.org/-32/how-hacker-news-can... (everything not ported to embedded-iframes yet, though.)
Maybe Reddit aren't advertising their embedded comments, so no one knows about it? A while ago I searched for embedded comments alternatives, and I found some abandoned projects (that had gotten super many upvotes here at HN), + Isso, + Disqus, + Talk, but cannot remember having found anything about Reddit. I wonder why they build it, but then won't focus on marketing it. Maybe they haven't thought about any way to make money from their embedded comments, yet. Hmm.