Historians were actually quite rare in the cultivation world. Partly because many sects and other powers curate rather edited versions of what could barely be called history in their favor.
Minor grievances could be turned into epic scandals that dictated the isolation or extermination of their foes. An innocent traveler is vilified to explain why they were stripped of all their valuables by an older cultivator.
It was not impossible to find greater powers that had more accurate records but it was still best to assume anything that sounded like it was too convenient for the hosting power was likely fiction or highly modified.
Still, some things come along that just flat out cannot be made any more absurd or less dramatic sounding if anyone tried. The fact all Celestials were turned into Fiends likely fell into that category but it was so massive and far reaching it was lost to time.
