And the thing here is that this is about how China is treating people in areas that are part of China, and have off-and-on been part of China for thousands of years. Some of them were part of China long before the proto-US was importing slaves from Africa.
So if the US wants to focus on human-rights violations, they arguably ought to focus on African-Americans. Some huge percentage of African-American men are, or have been, in prison. Far more than the Chinese are fucking with.
And fundamentally, it's not whataboutism. It's about realistic expectations. I mean, just consider what the US diplomatic response would be if China started organizing a UN conference on treatment of African-Americans. Pretty soon, as with China, it'd come down to "don't mess with us".