Definitely can appreciate the carefulness here. Imagine they just open the floodgates and now some random Mastodon instance on a $5 VPS is getting hammered with millions of activities because they followed an account with millions of followers on Threads, and now it’s federating millions of likes and thousands of posts.
Meta is trying to be a good fediverse participant here. They could just come in and crush the entire fediverse and be like “lol should have gotten beefier servers”.