The arguments I've heard about tracking etc are misguided and don't understand the actual risks.
Firstly, posts on the fediverse are already likely being consumed by advertising platforms like Facebook & Google. It would be trivial for big tech companies to setup relays that act as scrapers.
Secondly, the value in platform's tracking individuals is for advertising. There is no mechanism for these platforms to identify you browsing the we if your instance federated with threads. Your instance won't share cookie sessions etc with threads. It doesn't increase your exposure.
Thirdly, these platforms have the know how to deal with spam and they will be incentivised to share that tech with other federated instances.
Don't get me wrong, Facebook is an evil company. But I haven't heard a decent argument as to why them joining the fediverse is a bad thing. We always have the option to defederate in the future.
Change my mind.
The "user kick start" argument is interesting and not something I had heard. The fediverse does have active users which is valuable for growing a social media platform. However, Facebook would only need to convert 0.1% of it's users to the new threads and it would drawf the fediverse. So I'm not sure of that's their angle.
It's still a free userbase that they didn't have to grow.
They might not go down that route if they are successful from the beginning to establish a community. But they are still competing with Twitter and Bluesky, so they probably approached the instance admins to get an insurance that there would be activity from the start.
The last thing they want is to be the next Google+ (which they managed to beat). You have to guarantee buzz from the very beginning. After the metaverse flop, they cannot afford another one.