In today's political climate, when there are several groups trying to paint China as completely and uniquely evil in every sense, it is important to note that similar actions have been taken by other nations against them.
Additionally, from the opposite angle, by providing examples of times when other nations did similar horrible things, it shows both that the information is plausible and that China definitely knows what the result of this will be. This arguably makes it even worse, given that they have seen the way that lives can be ruined by addiction and overdose when supplied with these substances.
The poster you replied to said nothing except stating the fact, and too many people don't know their history and are apt to think that situations like this are only a part of modern history. If he brought up an unrelated bad thing someone had done, that would be derailing, but providing this information opens up some real discussion about how this happened and how to deal with it.
There are a lot of edgy athiests using all their time attacking trans people these days, because an unfortunate number of them are mainly athiests as a way to hate Muslims. And the ones taking the time to attack trans people have almost all joined the alt right, which has been responsible for a lot of attacks on innocent people.
They may not be killing in the name of atheism, but they have been in the same of "reason" and "defence of women".