One other possibility -- someone wanted make China look bad and made up the whole story... and had enough resources to paint a plausible enough story that Bloomberg believed it.
Also, why the need to make up a false story to make China look bad? There are plenty real issues that make China look bad. Before I get accused of some anti-China bias, the same could be said of the US. If I wanted to make the US look bad I'd focus on any myriad of foreign policy or domestic concerns that show it's failings before I concocted a story that would soon be discovered to be a fabrication.
Besides, it's more shocking if the Chinese hacked an American manufacturer rather than a Chinese one (where they would't necessarily have to "hack" anything, just compel the manufacturer to do it)