I appreciate that you're trying to maintain HN as a place for civil intellectual discourse, but I'd argue that people in these threads constantly implying China to be beyond criticism because "well the US does bad things too" is the actual "tired trope" here, not the people rightly calling out that fallacious logic and labeling it as what it is.
Sure, but we can't come close to moderating everything, or even seeing everything. I've certainly replied to many such comments on HN. If you see a particularly bad one that didn't get scolded, the likeliest explanation is that we didn't see it.
Whataboutism is relevant here. After all, one of the many reasons the right-wing elements became predominant in Japan in the first half of the 20th century is because Western powers were not willing to recognize it as an empire, even when they had done the same type of land grabs that Japan was trying to do.
edit: to clarify my clumsy statement a bit, I am not saying that criticism of America specifically is relevant here, or that America is just as bad (I agree that this is a ridiculous notion) but that the competing narratives and the past have direct consequences on politics. You can certainly see that on the discussions on climate change.