That's not what happened here though. For salaried workers everything you do that is related to your job is owned by your job. That's the default even if your contract doesn't state it. He may not have been directed by his boss to make the code for Box, but he did it with the intent of helping Box's business, as a salaried worker. That makes it Box's property.

But even if you are unconvinced of that, work was clearly done on it on company time at Uber, where it was deployed as part of Uber China's business infrastructure. That work is absolutely owned by Uber (with maybe also some claim by Box). Not owned by OP.