Most likely the Expeditionary Force took Beijing then settled down and founded an Empire of some sort because there was nowhere else to go. By 2277 thier descendants would be racially and culturally more Chinese than American due to intermarriage with the local population....a traveller from the US would find it hard to tell them apart from the native Chinese Empires around them.
The Shi managed to stay isolated enough in SF to maintain asian traits in appearance in Fallout 2. Of course, considering the canon ending for SF seems to be vines, it probably doesn't matter.