I've become less worried about this over time (this is something that's been debated on the Fediverse for quite a while... people have known about Threads and its goal of adopting ActivityPub for a few weeks).

I still have some concerns, but a few things worth noting:

• Threads launched this week, and doesn't currently federate with anything. We don't know when federation will be turned on, and it may never be turned on
• Threads already has many millions more users than the whole Fediverse. The Fediverse user count is a rounding error for Meta.
• Federating would be a nightmare to manage for Meta. They would have to decide which servers to federate with/defederate from. They don't want to audit all the fediverse servers, but they also don't want to let the fascist servers in. They can't apply their moderation policies to federated servers. Their users will likely get confused (e.g. some posts won't be visible to them, as often happens on Mastodon due to federation differences).
• The social links between the fediverse and Threads are likely to be limited. Threads is catering to an Instagram audience, celebrities/influencers and their followers, which couldn't be further removed from most of the discussion on the fediverse (including Lemmy). Have a look at this post to see some of the BS discussion on Threads: https://lemmy.nz/post/281577
• In my view, Meta has 0 interest of intentionally taking out the Fediverse by EEE. It's just too small and different from their core audience to care about.

So why is Meta even talking about federation? Most likely because of regulatory scrutiny. They are leveraging their market power of Instagram to create a Twitter rival, most likely with the goal of destroying Twitter. If they can say "but we're embracing open standards" that will help ease the pressure. They may not flick the AP switch until and unless they think they really need to. And there's a good chance they won't need to.

There's still a risk that they will federate, and that it will do damage to the Fediverse. We'll need to be vigilant if that happens, but I don't believe pro-active defederation is necessary (although a lot of Mastodon servers say they will do this). In any case the risk is probably low for Lemmy, but more of a factor for Mastodon.