This was during the Heian period of Japan / Tang dynasty of China though, and the spoken language in Xi'an would have been closest to modern day Min Nan. I'm not sure what other dialects pronounce tea as "cha", but modern day Mandarin originated with the Henan dialect during the Song dynasty with heavy influences from Mongolian in the Yuan dynasty and Manchurian in the Qing dynasty, and contains many sounds not present in any other indigenous Han dialects. Japan would have adopted tea long before. However, from what I know of the Japanese language, they have both archaic and modern pronunciations of many words, and perhaps their word for tea may be one of them with the modern form adopted much later.