Point being, Threads doesn’t need any other communities. People using Threads are those people who have never used reddit, and never would have signed up for lemmy. These people are also the same ones who don’t care about if their content is coming from a federated source, or just Threads.
And hence, defederation is a good idea.
Defederation is to protect us from them. You are absolutely right that they aren’t comparable to beehaw in size - now imagine if people here start joining the communities on Threads (not formal ones cus threads doesn’t have those), and we later decide to defederate, as some have proposed? Beehaw alone caused a massive clusterfuck, now imagine an instance with 10000x more users and power and concentrated community being let in?