Whether it is or not doesn't really matter. The Chinese economy has been doubling every 10 years or so since 1980. So if it isn't the world's #1 now, it very soon will be.
* Japan was (and is) effectively a US military protectorate. It could follow its mercantilist strategy up to a point, but it still had to capitulate to American demands (e.g. to open up its economy + stop putting downward pressure on the yen). China has no such restrictions and does not have to open up.
* Overall the Chinese economy is far bigger and the US is far more dependent upon it than it was upon the Japanese economy in the 90s.
China still looks like it may be facing a debt-crisis the likes of which Japan had in the 90s.