This is true - the bigger impact would be on Kbin instances that are both Threadiverse and Microblogverse facing.

However, if you go on Mastodon you can see Lemmy threads as posts which you can click through to the hosting instance and also boost (but not downvote). So for Lemmy if Threads.net federates, the biggest impact would be exposure of content to Threads users who then come in to Lemmy instances but not logged in or who could boost content and distort things.

For Kbin instances and also Mastodon it could mean being swamped with content from Threads.net.

Personally I do think overall the Kbin/Lemmy/Mastodon servers should probably not federate with Threads.net. The content appears to be poor and it could flood the fediverse with crap, when really it's still small and needs to grow organically. Threads.net is at 70m users already and rising rapidly, while Mastodon is at 8m (1.6m active) and the Threadiverse is more like 130k across Kbin and Lemmy. Mastodon/Kbin/Lemmy need time to establish what it means to be an independent federated social media network. They can always federate with Threads.net in the future - rather than it being Meta's choice, it should be the communities choice if and when they want to federate with a behemoth network.