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.
A instance can always generate bunch of fake user and use them to upvote their own stuff. With LLM I would imagine it would be trivial to make them sounds semi-human.
Unless fediverse implements some sort of smart contract system using proof of work/stake system, i don't really see much way to fix faking upvote. Even so, it is hard to make the implementation correct, and probably abusable (for example, most user only use a phone with 128G of storage, people can just buy couple 4TB drive and pretend each of them are 40 real users).
