It’s actually a real problem on reddit where people spin up fake users to manipulate votes. Reddit hasn’t published how they detect that exactly, but one way to do that is to look for bad voting patters, like if one account systematically upvotes/downvotes another. But you pretty much can’t without knowing the votes.
Whether a specific reason for defederating is a good idea depends on the instance IMO. I don’t think a “general purpose” instance should defederate on ideological grounds.
That said, they should defederate instances whose members are too disruptive. But right now we only speculate how hexbear members will behave. We will only know once they actually federate.