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’d have to check the upvotes, notice most of them came from one instance, look at the instance, check multiple users, and if they realize that these users were just created to get upvotes then they can defederate. However, it’s too big of an assumption that moderators will go through that kind of effort to validate all the upvotes.