Nothing will ever be truly perfect, it is rather an arms race where defenders construct barriers while attackers jump those hurdles - often easily but it does act as a barrier and some bad actors simply give up rather than do so.

In this case, PieFed has several relevant options, one being a per-community setting that only counts subscribed members of the community, which has the effect of reducing the impact of drive-by downvoters from All, but obviously won't stop a coordinated attack vector. The former scenario is real though, so the feature has actual benefits despite not stopping everything bad that could possibly happen, as it does improve the state of things incrementally.

Another such feature is the option to only count votes from "trusted" instances. This allows for finer-grained control so that e.g. you could remain federated with an instance, but not allow them to constantly brigade your content. Obviously someone could make accounts on trusted instances to do so, but the subscriber numbers being so low overall for the entire Threadiverse and for Piefed specifically seems to suggest that if it is happening, it is not a huge deal (yet). And the usual measures still apply, e.g. if an account only ever downvotes without ever posting or commenting, then it is likely a brigade account in (a not very decent) disguise.

Sometimes they will get more sophisticated, like repositing comics that seems an easy way to quickly generate many upvotes for the new account. But these seem to be shut down quickly... somehow, and anyway at that point whatever their intentions ultimately were, at least they were positively contributing to the Threadiverse community in the meantime, haha!