In that context the failures of the US (as it is de-facto the standard-bearer of political liberalism in a broad sense), have real influence.
When the Chinese look around the world and they see the state of the US on imprisonment, racial conflict, failure during the current covid crisis and so on, this strengthens the domestic control of the party and the alternative autocratic system the government is advocating.
That said, the parent poster isn't the US government, but a private citizen raising a valid criticism of another country. So why can't it stand on its own, without a big show of self-criticism first? Can we not criticize others until our own house is in order? Once that happens, the discussion turns into an argument over moral equivalence or lack thereof -- ie. "the US' crimes are just as bad as China's!" versus "No these things are of completely different degrees!".
This line of argument quickly becomes tiring and, I think it completely muddles the original point, which is often what is intended when hurling "what about...!" into the discussion.
But still, we should wonder if the criticism is motivated by something other than pure concern for human rights. You expect me to believe that Americans are mad at China strictly because they really, really care about the rights of muslims? With our record and our allies' records?
Or is it possible that this is just motivated by geopolitical interest?
Now, if we're talking about the US state department, then absolutely they're doing it for geopolitical interests. However, they're also reflecting the concerns of at least some of their citizens.
I'd also like to note that there's a moral equivalence argument to be made here. It's possible that China's abuses are actually worse than our own. Or maybe not. I just want to acknowledge that aspect of this argument, but I don't want to get into it because I'm not really informed enough to make it, and I'm certain that 80% of that impression is formed by skimming headlines and whatnot, which is not really a proper basis for debate.
I would be much more amenable to agreeing with the people that claim to be mad at China in the US if the solutions they proposed didn't give more power to US, that has no fundamental difference in foreign policy than China. Economically isolating China, for example, does absolutely nothing to help the treatment of Muslims in Xinjiang, but gives a lot more power to the United States. But if the solutions that were being talked about changed the balance of power towards entities that didn't wantonly abuse human rights, I would entirely agree.
Therefore, I don't think it's whataboutism. It would be whataboutism if the claim was that China actually respects human rights because the US is worse. But the question is different - it's whether we should economically isolate China on the pretext of their human rights abuses, or not. Saying that the party that benefits from this and that is pushing it is fundamentally just as disrespectful of human rights is not whataboutism, it's a question of whether the proposed actions will do anything for human rights at all.
It seems pretty clear to me that they won't.