Threads federation is mostly targeted towards Mastodon than Lemmy, so I highly doubt it will make much of a difference whether any Lemmy instance federates or not, since Lemmy is purely group based and does not federate well with even Mastodon to begin with as there is a huge difference in design philosophy. (Which means I can stay under the radar a bit longer.)
However, I don't think Facebook will stop at Threads, they are using Threads as a preliminary test, and if it goes well, I think the next step they could do is to get Instagram itself to federate.
So here is a thought: suppose reddit or Instagram are open to federation, would you say federating with them and getting all their content will be worth it?