As we do not know the original author, who says the Chinese stole it from the Americans? It could have easily been the other way around. In fact, there are any number of scenarios, from either side reverse engineering it/binary modifying, through to an unknown 3rd party supplying it to any number of international agencies. All a load of hearsay and codswallop.
There is a wiki article/numerous security articles about a Swedish worm, that clearly says it was The Chinese. I do love the way the security industry loves to cast blame and aspersions with no real proof of anything and still go to print with it too, much to the delight of the MSM who can then demonise at will.