and what precisely is the difference between a 'reporting ring' and an 'activist group', algorithmically?
Any kind of "mass reporting" should be blocked, regardless of the motivation. The only question when evaluating a report should be is it of a valid guideline violation or not.
The core problem here seems to be that Facebook doesn't have a human in the loop to filter the bad reports from good. Maybe they do have a human in the loop, but that human isn't trustworthy. In any case, their system clearly needs more work.
I think that voting rings should in fact be legitimate (because they show that a piece of content is popular within a certain group of people, which is exactly the purpose of upvoting functionality), while reporting rings that obstruct some individuals liberties should be actively banned.