I really, really doubt that this is going to be a concern. First, while technically Mastodon can interact with Lemmy, in practice how often does it happen? It's not zero, but it's not a lot, either, and I doubt that Threads will change that much because while it's a neat technical feature, link aggregators and micro-blogging platforms are pretty incompatible culturally.

And then we have to remember that we're talking about Threads normies. Do we really think that a bunch of Swifties and Kardasholes and other influencers are going to look at the absolute zoo of Marxist/Anarchist/Linuxist users on Lemmy and be like "this is the type of content I've been waiting for, I need to interact more with that community"? This reminds me a lot of neckbeards saying they wouldn't date Megan Fox because she has weird thumbs.

And then we have the whole thing with the actual fediverse and the tech behind it. There is still going to be no algorithm artificially inflating the popularity of what are thinly veiled ads. Meta has no mechanism for introducing ads into the Fedi. Lemmy is not suddenly going to be massively interested in the vast majority of content on threads and start upvoting to the moon.

And the dev team behind the fedi I would wager is going to prevent any sort of real technical takeover, so that means that at any point defederating is possible, and with basically no loss to the fedi.