The best part of the fediverse is that anyone can run their own server. The downside of this is that anyone can easily create hordes of fake accounts, as I will now demonstrate.
Fighting fake accounts is hard and most implementations do not currently have an effective way of filtering out fake accounts. I’m sure that the developers will step in if this becomes a bigger problem. Until then, remember that votes are just a number.
Don’t really need machine learning, just well built queries. Probably peak at the top pairs of users that comment on the same posts. If the same users are always commenting/voting on each other’s posts across every subreddit, it points to manipulation.