By the eleventh and twelfth centuries the Tibetan histories are telling a different story about him. In it he is an emanation of Avalokiteshvara, the bodhisattva of compassion, ruling a Buddhist kingdom. He converts his people, founds the Jokhang, issues a code of law resting on Buddhist ethics, and sends a minister to India to bring back a script so that the scriptures can be translated. That is the Songtsen Gampo of the Tibetan religious imagination, and it is the version most later writing repeats.
The early record does not support most of it. The Annals and the Chronicle, contemporary or nearly so, say very little about Buddhism in his reign, and the religious material they do carry belongs to the older ritual world of the Tibetan court rather than to an imported Indian one. The state sponsorship of Buddhism that is well documented belongs to the reign of Trisong Detsen, more than a century later.
None of that makes the later account worthless, and reading it as a simple fabrication misses what it is for. It records what Tibet came to believe about its own beginnings, and the belief did real work: it gave the Buddhist order a founding emperor and tied the dharma to the state at the origin rather than partway through. The atlas keeps both accounts and distinguishes them, because collapsing the two is how a devotional history comes to be read as a chronicle.