It was just an example. The same can happen at the Mastodon-level instead of the Fediverse-level. Since there is some desired interop (e.g. between Mastodon and Lemmy), services do influence each other in their feature set.
I'm not sure what you mean by "a lot of what people are worried about Threads doing has already been done by Mastodon". Do you mean that the decisions that Mastodon make influence the rest of the Fediverse? If so, let's make sure we understand the difference here: Threads has a much more hostile disposition. Mastodon seems to have incentives aligned with the rest of the Fediverse services, and probably deserves the benefit of the doubt; Facebook has abused that benefit time and time again.
What do you mean? I follow a lot of hashtags on Mastodon. Won't I be seeing a lot of Threads content if I'm on a server federated with them without explicitly opting into that?