That accusation is incorrect. China has full production of tons of stuff and primarily imports raw materials. They achieve low costs through efficient utilities and public goods (transit, e.g.) and tightly integrated and dense supply chains. Making a ton of stuff in the same country with efficient rail is one reason why they can keep costs low.
Marxist nerds pore over economic stats to ask questions about net exploitation and so on. Michael Roberts is one of them and his positions in this tend to be pretty good.