What I am talking about is a shared identity of a single civilization and an single people, which is something China have and no other Empire had, and especially the empires he mentioned in the beginning of Rome, Persian and the Arabs.
And you can't attribute this all to geography, I don't think Western Europe or Middle East had any easier geographic than Northern and Southern China, in fact I would say China's geography are quite harsh then compare to Roman empire and Person empires, and I would totally see that it is very possible that today's China would be like Europe today, split into many nations, each speak their own language and customs, but the fact is, it is not, and have not been for the majority of their history,
And if you want that if it was solely due to the pure willpower of the single warlords who with their willpower and determination is able to united all of China, then why haven't we see them in Europe or Middle East? because by this standards, there has been over a dozen strong will Chinese rulers able to do this over and over again in the time of chaos and break up, but not even a single such person from other civilizations all over earth? Because statistically speaking, this is almost impossible.
And I have said it before, there has been many times when China was fragmented in the past, and under those circumstances for any other people, it would have stayed fragment forever and it did, but not China, they always come together. You can't say this is all because of the will of a single warlord of geography.
Sun Zhongshan, Japanese, Mao Zedong certainly helped them to put a definition of what nationalism is, but way before them, the identity of a single people/civilization have long been deeply ingrained into the people's mind, consciously or unconsciously.
And I agree with your example of post Han and pre-Tang history, it was indeed chaos, but why don't you ask yourself, if it was pure chaos, what make them to unite together to became Tang after that? Why would it have not became like Europe? Where after the fall of Rome, the barbarians took over and the empire was forever lost? Under the circumstances for China, it would be entirely possible for the same outcome to occur, but why it didn't?
And the longer you go back in history, of course, the more fragmented the Chinese identity is, and the closer you go in history, the stronger the identity, so you can't just one instance in Chinese history and call it "Chinese" as the pure definitions of "Chinese", the Chinese identity have always been evolving, and the trend is the identity has becoming stronger and stronger as time passes.