I don't think Lemmy does either...? It pushes updates to subs that at least someone on the receiving instance subscribes to (at least that's how it worked last time I checked). That's why there are scripts going around for new instances to automatically follow a bunch of popular subs to populate the All feed.
I think Mastodon works in the same way with users, where it sends updates for accounts that someone on the receiving end follows. So if nobody follows you from Threads it wouldn't send any of your posts there.
Other people in that thread have pointed out that it isn't showing posts being delivered to Threads despite the block. That should be testable with other instances, but not Threads since it's not receiving any content from Mastodon at the moment. The concerning thing there is the user still being able to view content from people they've blocked, but that seems to be a bug if it's reproducible.