This is done because while currency fluctuations and inflation can jump and fall hugely overnight, an industrial base takes DECADES to build up and decades to destroy. China is planning for the long term.
If there ever comes a point where the US industrial base has been effectively destroyed and China makes everything, they can stop the treasury buying and start cashing in. Inflation in the US will go NUTS and the US will have to build an industrial base from scratch -- starting essentially in China's position circa 1980. Not a happy place.
China will have lost trillions following this strategy, but the US will effectively stop being a strategic or military threat if it is followed through to its logical conclusion.
Bitcoin is not a godsend to the Chinese government, as it creates a means to let capital exit the country (which they are desperately trying to prevent).