That is the sentence that follows the 3000 BC migration cite. What you're missing is that it doesn't say in there that any of those people, pre-historic or contemporary, are closely related to any modern Chinese. Of which there are several strains not just one. Tibetans are genetically a mixture of several different tribes of people including the red-headed white-skinned people who inhabited some parts of today's northwestern Chinese territory. The roots of their language is unrelated to Chinese as well. And there are differences among Tibetans themselves from the general populations of western, eastern and Lhasa Valley Tibetans. None of which are closely related to Han Chinese.
But even having pointed to all of that lack of relation to the Chinese it is still not the point. The claim that Tibet was ever part of China is a nationalistic claim meaning there was some sovereign control of one national group over the other. In 3000 BC neither China nor Tibet existed as distinct tribal units much less nations. From the time either of them did become distinct in culture and language only Tibet ever held control over some of China's territory and governance as a conquering nation.
“The Xia dynasty is thought to have run from the end of the third millennium B.C. to the middle of the second.”
http://ancienthistory.about.com/od/chinadynasties/p/032409XiaDynst.htm
This would indeed put the first known dynasty’s beginning well after the migration to Tibet. In which case, yes, China is playing dirty pool here. Their traditional culture, predating Communism, portrayed them as being at the center of the world, so it’s understandable to me that they felt little compunction at playing fast and loose.