Simplification: When americans and europeans buy chinese products then china has dollars/euro and we have physical goods. Dollars/euros can only be used to invest into usa/european companies or to buy usa/european products.

On top of that: Chinese financial markets are strictly controlled and the rates of return on safe investments like bank deposits are artificially kept down. Chinese citizens have very few trustworthy investment options outside of housing or sole-proprietor entrepreneurship, and the latter is only feasible for the politically connected.

Are non politically connected Chinese citizens sitting on any money to invest?

As I understand it, China doesn't allow speculation in anything other than real-estate and Chinese can't buy property in China (it's more of a grant from the state).

So there's a lot of money that can't be invested locally or in financial vehicles (ie, 401k).

There is the Chinese stock market, but it’s lost most of its credibility as being heavily vested in insider trading.