He’s saying the comment itself got flagged for simply pointing out an article being a union hit piece, which happens all the time here. As dang points out repeatedly, it’s not site moderation, it’s the users. There are enough active HN users that have flagging ability to wipe out whole discussion topics.

It wasn't the article itself that was flagged. The article was allowed to stay posted and there was some discussion about it.

But then someone commented, rightly, that the article was a union hit piece, and that comment was flagged and therefore removed by other users, despite it not breaking any rules.