This is the claim made by pseudo-historian Gavin Menzies. While I do believe some of the theories he put forward in his first book, 1421: The Year China Discovered the World, I think that he's simply kept presenting more and more over-the-top theories in each of his following books.
Irrespective of their historical accuracy, though, his books are great fun to read, if only for the great ideas they provide for an ocean-going role-playing campaign using The Celestial Empire.
This article is about Gavin Menzies, his books, and his theories. As I said, I think it can be a fun source of inspiration, especially if you want to incorporate Fúsāng (扶桑) as a Chinese version of the continent of America in your TCE campaign.