There is NO upside to federation with Threads. Zero.
That's a bit of an exaggeration. I don't think it will affect Lemmy at all, since it's a different type of posts. But in terms of Mastodon, it means a whole lot more content for Mastodon users, and a whole lot more people to follow. And also means that people can move to Mastodon while still having the audience that Threads provides.
Of course if you think that the downsides are worse than the upsides, then that doesn't really matter. But don't say there are NO upsides.