The mummies of Urum-Chi didn't have Han DNA.
Han Chinese are the dominant ethnic group in China, 93% of the population. When you think "Chinese" you mean Han.
First of all, how do you know that?
Second, the mummies of Urumchi were there before the Han showed up beginning around 100-200BC. They later mixed (amongst themselves and others) and are known today as the Uighyrs.