No rule prohibited mass downvoting.

Distinguish mass downvoting from indiscriminate downvoting. If most comments someone writes are terrible, there shouldn't be an issue with downvoting those bad comments (except perhaps if you also systematically hunt down all comments, including very old ones, making the average estimate of user's work severely skewed towards your own judgement). This would be a kind of mass downvoting, but not indiscriminate.

Who is to say (retroactively!) that a voter cannot rationally and sincerely determine that the best signal he or she can provide is one that is negative about a particular poster, based on generalizations about the content?

I agree. It would be great to also have a method of expressing disapproval of particular users ("This user has N friends and M enemies" or something), but indiscriminate downvoting of comments isn't it, it conflates different signals (quality of specific comments vs. overall impression about a user) and makes them less useful.