You note that you live in Hong Kong and chose China as your tax treaty country on your Form W-8BEN, but received a notification saying the proof of address and proof of identity submitted was not sufficient to claim benefits under the U.S.-China tax treaty. You asked why, if Hong Kong is a Special Administrative Region of the People’s Republic of China, the U.S.-China tax treaty did not apply to it.
According to the US Internal Revenue Service, the U.S.-People’s Republic of China tax treaty does NOT apply to Hong Kong. Unless you can provide a proof of address and identity in the People’s Republic of China or provide other evidence that you are a tax resident of People’s Republic of China, you may not claim Chinese tax treaty benefits.