If you follow someone on threads, they’ll only get your username, server name, profile pic, and server IP. That’s the same thing any remote instance gets. They’ll know if you like/share/comment on their content because you’d be telling them. Besides that, they can’t know anything. They only interact directly with your server, so they won’t even be able to tell if you see or click anything you’re subscribed to. In order to track you, they’d have to get info from your server (that isn’t collecting it to begin with). They also don’t have a reliable way to connect the sliver of data collected to an identity to serve ads to.

Really, the only reason they can collect as much about their users as they do is that they control their servers.