A few things here.

The first one that comes to mind is that defederation DOES stop your posts from going to Meta's platform when combined with the AUTHORIZED_FETCH server setting, while a simple user-level block may not. Depending on your server's settings, your posts may or may not be available on the open web where Meta could scrape the data - but this is still very different from them appearing in the feed or search results of, say, the transphobic, racist, or antisemitic groups that call Meta home.

This has serious implications for user safety and should not be overlooked. In fact, user safety is one of the biggest issues I have seen people mention when advocating for defederation.

Second: it's not yet clear if threads will allow their users to follow people on Lemmy or Kbin servers. But if they do, their users - including, for instance, the millions of followers of some big celebrity or politician - would be able to uprank posts and influence what you end up seeing. You might have LibsOfTikTok tell their users to brigade any posts critical of them, who knows. Meta's own algorithms could end up surfacing certain posts to their users, making the post rankings here largely a reflection of what Meta wants their users to see.

In other words, there's a lot more to the story than just 'blocking their content' when it comes to why you would want full defederation.

Here are a couple of blog posts that go into more detail around some of the data & privacy issues with federation:

https://privacy.thenexus.today/just-blocking-threads-isnt-enough/ discusses why defederation is much better than user-level blocking when it comes to protecting yourself from Meta

https://www.cacherules.com/blog/2023/6/resistance-is-futile-you-will-be-assimilated-by-meta/ discusses the things that Meta can learn about you via federation that they can't otherwise.