If some things get downvoted way out of proportion, and those downvotes come mostly from people in certain instances, and instances have the power to block each other, it stands to reason some instance out there managed to block the instances that contribute the most downvotes, causing them to have mainly upvotes. Right?
As far as I understand ActivityPub (which, admittedly, is only very rudimentarily), downvotes are an action like any other: you perform them on your instance, then the instance sends them to the instance the community is hosted on, then that instance sends them to all instances which have users who are subscribed to it.
An instance which has disabled downvotes will ignore all of that, i.e. will not store downvotes or forward them to anyone. If you look at beehaw.org you can see all posts and comments are displayed with zero downvotes, even if you view them from another instance.
That answered my question exactly, thanks very much!