That's exactly what vouching is for: https://news.ycombinator.com/newsfaq.html#cvouch.
coryrc already pointed this out but I want to hammer the point :)
(Users have already vouched for the comment so it's no longer dead.)
To flag a comment your account needs a minimum score, I'm not sure, but I think it is 50 (or 100 or something).
To flag something you click on the timestamp on top of the comment (typically something like: <x> hours ago). This opens a view of only that comment and replies to it. If you have enough reputation/points/karma, you can now see a flag link on top of the comment. If you click flag it ends up in a moderation queue. If multiple users click flag it might bypass the moderation queue and become flagged immediately without further intervention from mods.
(No one flagged it.)