Yes, the UN is an authority on what most nations once thought, if that's all that's being argued for. At one point there was an understanding that Taiwan is to be de facto independent and de jure a part of China. Now China is pushing back on the de facto independence a bit, and the US is pushing back on the de jure union with China a bit.

The UN proper is not an authority on anything other than what clubhouse members publicly agree, and can't otherwise be used as an argument from authority, which is how this came across.