The Dunhuang records indicate no such thing.
They don't indicate a strongly Buddhist culture by any means. Let me cite an immediate example that comes to mind:
Songstän Gampo's reign was marked by continual violent expansion and conflict. He inherited the throne from his father Namri Löntsen between 625-627. In the days before his father's reign central Tibet had been controlled by a confederacy of clans including the dBas, Myang and mNon. Namri Löntsen had obtained oaths of allegiance from these tribes with which to expand his territories to the north and east of Lhasa and to Kong Po. Later the Dagpo rebelled and had to be reconquered. Following Namri Löntsen's death these problems mounted. The histories differ on what later transpired.
The Tun-Huang Chronicles
state the following:
...the paternal subjects rebelled; the maternal subjects revolved. ... The father gNam ri was given poison and died. The son Srong btsan firstly wiped out the families of the rebels and the prisoners.
Meanwhile Butön Rinchen Drup (Wyl. bu ston rin chen grub) (1290-1364) relates the following:
The History of Tibet Volume I, 338.
...Thirteen years of age he ascended the throne and, brought under his power all the petty chiefs of the borderland who offered him presents and sent their messages (of submission).
The former would be the earlier history from a time before Buddhism had thoroughly penetrated Tibetan culture.
Basically, Tibet under the Yarlung dynasty was hardly a Buddhist culture, nor was Buddhism that widespread as later people would believe when certain kings were understood as being devout Buddhists who introduced the Dharma into the country.
Furthermore, the Chinese as far as my readings from the Tang go seem to have been unaware of much Buddhism being in Tibet. The later histories of Tibet tell a different much more Buddhist-friendly story, whereas more objective historical scholarship would have one believe, in fact, Yarlung Tibet was hardly Buddhist.