Of course a bad actor can set up their own instance and just create thousands of fake bot accounts and do the same.
A competent admin would then just defederate from them. Easy. But now throw in that all kbin instances look like bot fests and what do you do? Maybe what lemmy.ml have done and just block kbin useragents at the firewall.
Having an aggregate account that just sends totals could work, but then vote brigading just became even easier. What's that aggregate bot? Did you just send a vote ratio of 300:1.9k for this comment? Lovely.
It's a very hard problem to solve and I'm not sure it's doable. The only thing keeping ActivityPub together is the fact that it's so transparent and bad actors are easily spotted and blocked. As soon as you muddy the waters the primary benefactor is the bad person.