What is not in doubt is that Tibetan was first written in the imperial period, in the seventh century, and that the script is Indic. Its letter shapes, its ordering and its structure as an abugida, in which each consonant carries an inherent vowel modified by marks written above and below, all come out of the Brahmi family. Where in that family is the live question. Proposals have pointed to Gupta Brahmi as transmitted through the Indian northwest, and to the scripts of Khotan and the Central Asian oasis towns, with which the Tibetan court had close and continuous contact.
The script also carries a design decision that no simple borrowing explains. It records consonant clusters, prefixes and suffixes that Tibetan speech had at the time and that later pronunciation dropped, which is why Tibetan spelling now looks so unlike Tibetan sound. Somebody analysed the spoken language carefully before writing it down.
So the atlas records the attribution and marks it disputed. A script was designed, and it was designed well. Whether the designer was a minister called Thonmi Sambhota, several people working together, or a court scriptorium over a generation is not something the surviving evidence can settle, and the tradition's answer is the only answer anyone has been offered.