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: To clarify, not just YOUR instance admin gets this info. This is ANY instance admin across the Fediverse.
This is what lemmy.world tells me when I want to delete my account:
“Warning: this will permanently delete all of your data from this instance. Your data may not be deleted on other, existing instances. Enter your password to confirm.”
Edit: So if we want to own our data we should only post, comment and vote within our own instance or just keep in mind that whatever we do on other instances might be there indefinitely.