Very little from the first Islamic century survives in a form historians treat as documentary, and this text is the striking exception. The reason has to do with what it does not say.
A document forged later to serve a later purpose would be expected to match later doctrine. This one does not. It uses the word umma for a body that includes non-Muslims, a usage that later Islamic law does not support. It gives Muhammad an arbitral role rather than a sovereign one, and calls him the Prophet without the titles that quickly became standard. It names Jewish clans as constituent members of the community at Medina, which sits awkwardly beside the narrative of their later expulsion and destruction that the same biography goes on to tell. It refers to pagan Arabs among the parties. And it contains archaic administrative language that later transmitters evidently did not fully understand, since they preserved it without smoothing it.
Those features are hard to explain as invention and easy to explain as survival. Julius Wellhausen argued for its authenticity in the 1880s and the position has held broadly since, though scholars differ on whether the surviving text is one document or several agreements of different dates run together, and on exactly when it was drawn up.
What remains contested is its afterlife. It is regularly cited in modern argument as a charter of pluralism or as the first written constitution. Those readings are claims about the present, not findings about the text, and the atlas keeps them apart.