I believe it absolutely depends on who you ask. Of the untold quintillions of humans alive in the grimdarkness of the 41st Millennium, a solid majority are ignorant, uneducated in all but the most practical vocations, or have wildly different viewpoints/inconsistent knowledge/inherent viewpoints which go from "clashes completely with reality itself" to "actually hilariously the absolute truth, from a certain point of view".
When the Emperor was enacting his little power fantasy, he did not put much thought into informing anyone of anything of actual importance, barring a select few, because he did not trust them with anything. And being one of the oldest humans alive, he would probably be the one with anything like an objective historical record of events along any timeline. You can call everything before the Great Crusade a literal blank spot with only scattered, distorted records blended through religious histrionics and recovered Dark Age databases.