Showing that China is doing similar things to the US doesn’t seem like a strong argument if the thing the US is doing (in this case indefinite detention without trial in a horrible prison) is bad. Is the idea that post-federation there’s users who don’t view the US as doing bad things?

The problem is that liberals are operating on "Our country (the US, UK, a European country, etc) is better than China because of these reasons, China bad, 100 million dead" and so the idea is to first go "Actually, China isn't doing anything worse than the United States is doing" and then later on go "...and, in fact, the United States is the one that's by far the worst." Basically to cushion the blow of having their worldview swept out from under them.

So the first step is to go "Oh, is China bad because they imprison people for revealing state secrets? Then look at all these people in your own countries that have done the same."

And then the second step is to go "And, in fact, China has a lower number of incarcerated people than the United States despite having almost five times more people."

of course, then they start blubbering about "buh buh buh, they're lying and a-actually have trillions in prison and they're killing them and xi is personally beating them because he's evil and a monster and the CCP they're bad and they--" but the seed of doubt has still been established