If I understand Meta's announcement here, posts from outside of #Threads are not visible in Threads at all. Not yet, anyway. Currently, information can flow from Threads into the Fediverse, but not in the opposite direction.
The "our phased approach to the Fediverse" part of this indicates that Meta eventually plans to allow two-way communication between Threads and the Fediverse.
https://engineering.fb.com/2024/03/21/networking-traffic/threads-has-entered-the-fediverse/