Meta announced that users on Threads will be able to see fediverse replies on other posts besides their own. In addition, posts that originated through the Threads API, like those created via third-party apps and scheduling services, will now be syndicated to the fediverse.
It is a different level of scale, mastodon has about 1 million users spread over a bunch of instances. Threads has over 200 million users on one instance. also due to the network nature of social media the amount of connections and messages sent through those connections can scale exponentially with the amount of users.