You're right that it's a bit trickier than I thought. It blocks the shared content: images and other content hosted at lemmy.world can be blocked from https://kbin.social/d/lemmy.world/ , but it will not include links to external sites, nor will it work for text posts. Blocking lemmynsfw.com worked fine for me, but that's of course because it's an image-heavy service.
Whether or not it will work for threads.net off the bat I guess then depends on how Threads interacts with the fediverse; whether it merely shares a link content stored locally, or whether it distributes the content in its entirety.