Tibetan writing has a beginning that can be dated to a century and a story that cannot be taken at face value.
The traditional account, transmitted in Tibetan histories, credits the minister Thonmi Sambhota, sent to India by the emperor Songtsen Gampo in the seventh century, with bringing back a script and composing the first grammatical treatises of the language. The account is old, it is universally known within the tradition, and the two treatises attributed to him are still the basis of how Tibetan grammar is taught.
Modern scholarship does not reject the seventh century. The earliest Tibetan inscriptions and documents are consistent with a script created or adapted in that period under imperial patronage, for the practical purposes of a state that had just acquired an empire and needed to write. What scholars treat with caution is the rest: the figure of Thonmi himself is attested only in much later sources, the surviving grammatical treatises show signs of later composition, and which Indic script served as the model is disputed rather than settled.
The atlas records both parts honestly. That the script descends from the Brahmi family is not in doubt and is stated as such. Which script of that family stood immediately behind it, and how much of the traditional account is history, are open, and the entries say so rather than choosing.