auster@thebrainbin.org you're right, the experience is definitely sub-par, and it's because both Mastodon and the Threadiverse use the Announce activity for different purposes.
Mastodon uses it to boost posts to the feed (overriding existing logic re: replies being suppressed), and the Threadiverse uses it to keep different instances up to date.
It could be made better in that when a group actor announces a reply, Mastodon doesn't promote it to the feed. Not sure if that's easy to do.