The evidence for the movement and the evidence for the man are of different orders, and keeping them apart is the whole of the historical problem.
For the movement there is a contemporary secular record. The Sanguozhi and the Houhanshu describe a religious polity in Hanzhong, on the road between Sichuan and the north, governed by Zhang Lu, who ran free hostels on the highways, punished offenders by requiring road repairs, and pardoned a first and second offence before imposing any penalty. In 215 CE he surrendered to Cao Cao, who did not destroy the movement but enrolled it, ennobled its leadership and moved its adherents north, which is how a regional Sichuanese cult became an empire-wide church.
For Zhang Daoling himself there is much less. He appears in Ge Hong's Shenxian zhuan and in later Daoist scripture as a transcendent who ascended in daylight, tested his disciples, and left texts and a sword. No source contemporary with him names him. Historians therefore divide: some accept a real founder behind the hagiography, on the ground that a grandson who governed a functioning theocracy implies two generations of something, and others treat the founder as a figure retrojected by the movement to give its revelation an origin.
The atlas records that split rather than settling it. What is not in doubt is that people acted, and still act, on the covenant they trace to him.