I mean, for now.

Mastodon, through its dominance is already shaping what the protocol is and isn't. For instance, the Server to Client API that mastodon runs is of its own making and design and just about every microblogging app relies on it such that any other platform tries to mimic it. It's become a de facto standard. Should mastodon change their API, many other platforms will feel compelled to follow suit. There are now voices calling for it to be standardised. BUT ... talk to people working on the actual protocol and they'll say they hate this because the protocol already has a standard for this and it should be used instead ... and app developers will basically say "well, everyone is using the mastodon API already ... why would I use this thing no one knows about".

Threads/Meta can do exactly the same thing over time. And once they have control over how some parts of the fediverse operate, which they will have by having "the standard" and the dominance of users to force people to comply ... then they can influence what is and isn't in the standard to suit their purposes (think surveillance and ads) and even add things that only work on Threads, which of course will presumably attract more users (as Threads is already huge).

More abstractly ... "our turf" here isn't the protocol. The protocol is over-emphasised as some magic element that makes everything here work. It's just a tool. The stuff that actually makes the fediverse work are all of the software platforms, such as Lemmy and Mastodon, that provide the actual social media we use. And they just use the protocol. It's the quality and design choices of these platforms that are "our turf", and these depend very much on the developers and the users and their motivations/desires. Threads is big enough that it can distort the network of motivations. An example ... There's a mastodon mobile app (Mammoth) that is the only one to implement a recommendation/algorithmic feed. One of their key motivations (they've stated so publicly) is to be ready for when Threads joins the fediverse so that their app can attract Threads users. They also run their own mastodon instance, which I can only presume they'd be happy to modify with their own features.

Another way they can exert influence is through altering the way moderation affects the fediverse. Moderating what comes through from Threads is likely to be onerous. It alone will be a reason for some instances defederating. But some instances will want to stay connected to the large userbase of Threads, and will tolerate some of the garbage coming through. The net effect will be to splinter the fediverse between those that can't and those that can tolerate a lower average quality of user/content. Such a hard splintering wouldn't occur if all of those users were spread out amongst more instances instead of coming from a single source/instance whose size alone attracts disproportionate interest and gravity (to the point that this discussion happens again and again).