At least read the explanation of that rule first, would you? There you go:

If we determine that you're e.g. following a particular user around and leaving insulting comments to them, we reserve the right to delete those comments.

Your leading OP title, including the phrase "mass-downvote harassment" is insincere reasoning, because it is circular. It has never been established whether mass-downvoting should always be considered "harassment". You'd consider it so. I don't. Come now, be so courteous as to assume other people have reasons for their behavior.

Not even the wiki, which does include an example, makes mention of mass downvoting even though the topic has come up many times. The reason for that is not "well, we can't list everything, we don't list hacking a server, for example". That would be a ridiculous argument. One is using established feedback mechanisms, one isn't. New rule: You must always give reasons for each and every vote, otherwise you'll be publicly shamed for harassment.

Downvotes are a user's individual and private choice. He/She can use it to confer whatever message he/she so chooses. Don't like it? Make a rule against it. Such as an upper bound on allowed downvotes. Oh wait, such an upper-bound has already been implemented? And it doesn't disallow downvoting most of a user's comments? Maybe your moral intuitions on the matter aren't as general as you'd like them to be.

Signing off on the topic, though I'll leave you the last word, if you so choose.