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.
It’s an easy target but also a near worthless one. What does the CCP gain from getting a bunch of terminally online Linux nerds on their side? No one making real decisions that could effect China is on here.
4chan is also a soft target but you don’t see parties or governments investing in propagandizing there because it’s full of basement dwelling incels who can’t do anything besides maybe shoot up a school.