They’re private in the sense that there isn’t a corporation stealing your data without your knowledge, selling it without your consent, whoring you out for ads against your will, and/or making your experience shittier to manipulate you into buying their paid features. These alternatives offer a much more pure experience for the typical user. Things like comment and vote history being public is just a part of the design of the forum, they’re not tools to farm your data.
Is that true though? Any given instance could be running their own data collection.
They could, but they’re pretty much guaranteed to not be as bad as the big corporations because they lack the resources and know how. The fact that you could also host your own instance is a huge plus considering that you could basically ensure that your data won’t get collected.
Even if you’re self-hosting, you still have to send your data to any instance hosting posts or comments you’re interacting with, otherwise you’ll be the only person to ever see your own posts and comments.
No other instance will be guaranteed to have a complete profile of you, though.
Things like comment and vote history being public is just a part of the design of the forum,
Or, to check my understanding, make the structure of the Frediverse actually work, right?