The only problem I see with this conclusion is that the Chinese claim the Koguryo kingdom as part of their history as well, which makes this recent finding irrelevant:

A crown ornament bearing a striking resemblance to a Koguryo crown has been unearthed at a royal tomb of the Balhae Kingdom, while an epitaph states that the buried individual is an empress. The find by Chinese government researchers directly contradicts claims by the country’s Northeast Project that the Balhae Kingdom was a province of the Tang Dynasty.

The Jilin Provincial Institute of Cultural Relics and Archaeology and a team of researchers from the Office of the Yanbian Korean Nationality Autonomous Prefecture Commission for the Preservation of Ancient Monuments excavated 14 ancient tombs in the Helong region of Jilin Province dating back to the Balhae era. They published their findings in the latest issue of the journal “Chinese Archeology” published by the Institute of Archaeology at the Chinese Academy of Social Sciences.  [Chosun Ilbo]