Tibet as part of China has never been universally accepted by nations, so that's a lie.

As for the historical connections between Tibet and China, it woudl be worth doing a little bit research, read a book such as Thomas Laird's The Story of Tibet: A Conversation with the Dalai Lama that is a very good summary of the complicated relationships.

Basically if you have a culture that does not speak Chinese, do not use the Chinese alphabet, don't cook Chinese food, neither use Chinese clothes, nor Chinese architecture, and still claim it's part o the Chinese motherland is somewhat absurd, me thinks. Or a sign what the Chinese want to do with all the other Asian cultures.