The procedure for international to international connecting luggage in the US is confusing. Some airports support luggage transfer without customs, others don't. Sometimes it depends on the airline. But I think ATL is one of the few airports that does support international luggage transfers without collection and customs.

As for the OP, my experience is that luggage must be collected and rechecked at the point of entry into mainland China, just as in any country (such as the US) where domestic airports don't universally have customs. Cabin crew sometimes announce this on board, sometimes not.