>You're conflating racial and ethnic diversity in ways that are distinctly western or even really US-centric.
Not at all, these are pretty universally agreed upon by global sociologist.
>Europe is extremely ethnically diverse, as is China
Indeed, Europe is extremely diverse, much more so than China.
>they just don't use skin color as the primary ethnic marker the way that is commonly (and still incorrectly) done in the US.
Neither does the US. Skin color is not a primary ethnic marker. No one versed in sociology in the US considers a Black American ethnically similar to an Eritrean. Nor do they consider a Ukrainian ethnically similar to a White American.
>The 90% Han Chinese number isn't especially useful because it's comparable to the way that most of Europe has historically identified itself as the successor of Rome.
It is extremely useful because the subgroups specifically make these claims: "Modern Han Chinese subgroups, such as the Cantonese, the Hakka, the Henghua, the Hainanese, the Hoklo peoples, the Gan, the Xiang, the Wu-speaking peoples, all claim Han Chinese ancestry pointing to official histories and their own genealogical records to support such claims."
Germanic people do not make the claim to be of Roman or Mediterranean ethnicity nor origin. The languages are vastly more varied in Europe versus China, where 70%+ speak Mandarin.
Additionally, Han Chinese are much closer genetically than Europeans. Italians, Brits, and Estonians have much more varied genetics compared to Han Chinese.