Currently, I think there are two main branches of ActivityPub implementations: Microblogs(Mastodon and its forks, the microblog portion of kbin), which are user centric, and group based aggregators(Lemmy, Kbin, peertube, future Pixelfed), both of which are valid implementations, however, they don’t really work well with each other.

So, I believe that the threat of Threads to Lemmy instances is really overblown for the simple reason that there is no way for a Lemmy user to browse microblog contents through federation to begin with, whether it be Mastodon or Threads.