Well, underpaid by what metric? If each Chinese laborer produces $100/day in profit but only gets paid $10/day, morally they're being underpaid. And the fact that so many companies are still moving operations to China is itself evidence that on average more profit is allocated to capital holders when they use Chinese labor over, say, Californian.

But even if you take liberal/market ideology at face value, they're still being underpaid. To have a free market you've got to have free people. If you agitate for more workplace protections or (god forbid) try to start an independent labor union, getting shot or sent to a prison camp in the far western provinces of China isn't out of the question. This decreases labor's bargaining power relative to what it would be in a free market, which decreases wages.