How would China having "so much to gain" lead to Snowden knowing more than he leaked? Maybe your opinion is unpopular because it contains very little logic.
Still, same question, how would it lead to him having documents he handed off? There is, after all, a big difference between China wanting them badly, and Snowden wanting China to have them badly. Just because they wanted him to still have the documents likely had little to no bearing on him having them.