Edit: obligatory explanation (thanks mods for squaring me away)…
What you see via the UI isn’t “all that exists”. Unlike Reddit, where everything is a black box, there are a lot more eyeballs who can see “under the hood”. Any instance admin, proper or rogue, gets a ton of information that users won’t normally see. The attached example demonstrates that while users will only see upvote/downvote tallies, admins can see who actually performed those actions.
Edit: Obligatory RIP my inbox.
What about private messages? I assume the person running the instance can read all private messages.
You don’t have to assume, most instances straight up tell you that the dm function is not secure and to use other means for private messaging.
This is true for almost all social media platforms, though. Including Reddit.