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.
It would be equally as easy to lie either way. Instead of reporting 1000 anonymous upvotes, report 1000 fake upvotes from fake users or users who didn’t actually upvote. Any instance that gets caught abusing the protocol would get blocked pretty quickly though. Even though it’s theoretically possible, I’ve never seen it happen.