A few points to make on this topic:

1. There is presently nobody to defederate from: They aren't in the fediverse yet, and while we can guess they'll use threads.net as their domain, that's all it would be - a guess. There is no point preemptively defederating from that domain when we don't even know it's what they'll use in federation.
2. Defederating is trivial from the instance level. Just type their domain to the "blocked instances" setting and it's done for everyone. This move can be made at any time in under a minute.
3. A change to the behaviour of aussie.zone this large will likely be passed by the users in some way. It's not a thing to undertake lightly.
4. The new Lemmy allows users to block instances for them personally. If you don't want to personally interract with Threads, just add them to your block list and it's done. You won't see anything from that instance. It'll also mean you won't see replies to their comments made from here. ^*^

^*^ You can't actually defederate from them yet, as the "block instance" UI searches for federated domains and won't permit you to add threads.net as that domain is not federated. You'll need to do this if/when they actually federate with the rest of us.