Privacy can mean different things in different contexts.
Some peoples' thoughts go first to sharing content with a restricted audience. ActivityPub isn't good at that since the admins of every server involved can access the content. That's also true of centralized social media, though sometimes the admins of those services seem farther removed from users' social lives. E2EE chat like Matrix and Signal are good options for that use case, and there has been work on adding E2EE options to some ActivityPub software.
I usually treat social media as public, so I'm not concerned with restricting access to things I share that way. I am, however concerned about service providers monitoring behavior like how long I spend looking at a particular post, or trying to track my browsing habits on third-party websites. Fediverse projects do not normally include those kinds of behaviors, and it would be scandalous if a service provider added them.