Literally everything in your post is wrong.
1. No popluations are physically distinct from their neighbors. North Africa, Middle East, and Central Asia all have populations with people who could pass as "looking caucasian" from right across the "border of christendom".
There are Caucasoid populations outside Europe but generally in small numbers easy to ignore. Most Afghans are indistinguishable from Europeans but they can be treated as exotic exceptions. This is nothing like Mongoloid people who populate multiple continents sharing similar features.
2. Regardless of the distinctiveness of differences between neighboring populations, what really matters are perceived differences. Some perceived differences are easier to notice when you know the two neighboring peoples, but from an outsider perspective "they're all the same to me". So to a European, a Tatar and Han might have "Mongoloid features" but to East Asians, Irish and Polish are indistinguishable.
Objectively Irish and Poles are more similar than Tatars and Han.
Furthermore, some perceived differences even between groups on different continents are virtually imaginary, such as the "white" skin color of East Asians and West Europeans. "Yellow Skin" was pretty much invented to give Europeans who were ignorant of Asians a reason to see them as different, other, and thus fearful and untrustworthy.
Sure but what we call race is not a single distinguishing feature. It's an effective short hand for telling which part of the world someone originates from and to project stereotypes onto them.
In the modern world if you see a white person you can reasonably infer they are better off than most of the world's population. I'm not sure what a Chinese person can infer from seeing an East Asian looking person in this hypothetical world. If they can't infer their wealth, education, status. Then I'm not sure how projection of stereotypes would work.
This leads to -
3. Regardless of differences and similarities, ways will be found to racialize groups based on the political and economic needs of Empire-building. Other Asians with narrow eyes will be seen by the Chinese as inferior regardless of how unclear the dividing line is, just like how Europeans see Middle Easterners as different even though when shown real-life pictures of Syrian people's faces, Americans didn't realize they're weren't European until told so.
I agree the Chinese will probably see most other similar looking people outside the Confucian sphere as inferior, just as Europeans see Syrians or Afghans. The difference is scale. There aren't that many Syrians and Afghans compared to Europeans and North Americans. If most of the population of the third world looked like Europeans, then racial attitudes would be very different don't you think?
Civilizational developments don't really matter. They can be explained away, as "Those people couldn't possibly have built those monuments, it was obviously some transient master race which slipped into obscurity from allowing miscegenation with the dirty locals." This was a real attitude towards the achievements of Indians and Africans. Or alternatively the "They were once great but fell into becoming decadent" for the Persians.
Europeans learned of the greatness of Africa's wealth in the Malian and Songhay Empires through Muslim merchants. They did know about Africans having rich and complex societies, but then conveniently forgot about that when it was time to sell them into slavery and colonize them.
Basically, any "evidence" can be subordinated to the political needs of those who wish to exploit others for economic gain.
I agree that people can demean other people's achievements. Certainly it is possible these hypothetical Chinese could belittle the civilizations of others. But that is a distinct phenomenon from assuming yourself is superior because of your phenotype. That would mean you believe everyone that shared your phenotype is also superior which would make no sense to the Chinese in this world. It's possible they might have racial constructs like the Japanese and Koreans who believe they are distinct because of some creation myth. But just as the Yamato race does not include the Chinese, a Chinese racial supremacist would be less likely to group people based on phenotype.