You probably already know/have done this but just in case (or if someone else reading along isn't aware): if you click the timestamp "<x> ago" text for a comment it forces the "vouch" button to appear.
I've also vouched as it doesn't seem like a comment deserving to be dead at all. For at least this instant it looks like that was enough vouches to restore the comment.
I don't know the specifics of this, but vouching "against" the hive mind leads to your vouches not doing anything any more. I assume that either there's some kind of threshold after which you're shadowbanned from vouching or perhaps there's a kind of vouch weight and "correctly" vouching (comment is not re-flagged) increases it, while wrongly vouching (comment remains flagged or is re-flagged) decreases your weight.
We sometimes take vouching privileges away from accounts that repeatedly vouch for comments that are bad for HN in the sense that they break the site guidelines. That's necessary in order for the system to function—you wouldn't believe some of the trollish and/or abusive stuff that some people vouch for. (Not to mention the usual tricks of using multiple accounts, etc.) But it's nothing to do with the hive mind and it isn't done by the software.
It wasn't downvoted - rather, the account is banned (https://news.ycombinator.com/item?id=42653007) and comments by banned accounts are [dead] by default unless users vouch for them (as described by zamadatix) or mods unkill them (which we do when we see good comments by banned accounts).
Btw, I agree that that was a good comment that deserved vouching! But of course we have to ban accounts because of the worst things they post, not the best.
