Can't copyright an idea, and software isn't patentable as far as I know. Trademark isn't being infringed. So what does that leave? Design rights maybe? I haven't seen Threads, but there's nothing particularly unique about Twitter's design that you can't find in tons of other apps already, as you say.
Twitter's moat was its brand and userbase. Both of which Musk has spaffed away. Deliberately weakening the brand with "Titter" jokes, and alienating users.
Twitter CEO Linda Yaccarino said in a tweet on Thursday that while the platform, which has previously reported about 260 million monthly users, is "often imitated" it "can never be duplicated".
Twitter is claiming that Meta hired all of the people that Twitter laid off and have stolen their "trade secrets". Considering that Threads uses ActivityPub I don't believe their claims have merit.
In my opinion, Elon sees this as the only opportunity to recoup the billions he lost by purchasing Twitter.