Seems you ignored half the sentence, so I'll repeat with emphasis: HN has a large Chinese user-base __who will be quick to downvote and flag threads and comments critical of China__.
Heck, it could be one guy who has 1-50 high karma accounts that do all the downvoting and flagging. He can then monitor https://news.ycombinator.com/newcomments for threads/comments containing certain keywords to get a notification.
There's no denying that the downvoting and flagging is taking place because you can see the same pattern in all threads that are negative towards China. Dang himself also have a history of knocking threads critical of China off the frontpage.
Dang isn't Chinese (his username is not a romanization of 党, but short for Daniel Gackle), but as you say he occasionally takes threads off the front page. Usually because he thinks they'll only lead to repetitions of the same old flamewars instead of significant new discussion.
There's no reason to suppose that a majority of people participating in China-related threads and voting (in either direction) on comments is Chinese. It's likely that they have some strong opinions on the topic, but ethnicity is neither necessary nor sufficient to develop such opinions.
