I agree with the point, but if you take assimilation as a function of two (for simplicity) cultures that produces another culture as a result, the assimilation of groups into the Han identity took two (somewhat, usually at least a little adjacent due to prior exchanges) cultures and put out a culture that was much closer to the pre-existing Han culture than the other.
In this sense, this process of assimilation (which certainly happened in many times in history) still is not "molding," which was the point of my post: what is now the Han cultural identity has existed for some thousands of years, and while now many genetically distinct groups fall underneath the cultural identity (either as part of the original Zhou-period umbrella, or later assimilations), this conglomeration was not "molded." "Molding" suggests some mastermind taking large and distinct groups and forcing them in some manner to adopt a shared culture and identity; this is not what happened.
I also would not really say that these genetically distinct groups are distinct ethnic groups in the same way that regional dialects are not distinct languages. What I wrote regarding mutual intelligibility, legibility, and shared cultural values holds true (although with different numbers for how long it's been the case) for all the various genetically distinct groups in China. Perhaps my notion of ethnic group differs from the conventional notion. I personally feel that any efforts (generally from the West) to point out these small cultural differences or genetic differences as issues of division in Chinese culture and society are efforts to generate tension where there is none.
@taxiya
I think you can Europe as an counter-factual example.
Suppose after 2000 years of mixing, everyone in Europe now identifies as "Roman" or "European".
There is now a single national language, such as Latin or English.
You still identify with your province eg. France or Germany.
And you have local language dialects such as French or German.
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The existing peoples have largely remained in place, but you can see the assimilation.