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.
They could also track the IP address it came from and sell the data if anyone wanted it. I’m sure there are corroborating databases out there for IP address to people lookups
edit: One should also consider that Facebook and Twitter actively sell whatever data they have on you, along with the ability for people to put things in front of your face in all the places you tend to read things. At least on lemmy, you’ve got a volunteer admin that might have scruples.