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’s a lot easier to fake a hash than a username. If I’m an instance owner and I suspect another instance of this, I can grab a random username and check their post history. Pretty easy to see rampant fraud that way.
If you’re putting something out on the internet, even upvotes or clicks, expect it to be public.