But then, so was Archeon. A lot of things had to go wrong in just the right way for him to even join Chaos, nevermind rise all the way to Everchosen.
Accordingly to the book on him, which should be take with a grain of salt as we don't know who would be told the accurate details and then tell of them, lots of things wouldn't have gone wrong, and Be'lakor basically save-scummed by telling his past self how to make sure they did when what his original plans were failed.
That being said there have to be some things that are more trustworthy then others. When the lizardmen army book talks about the coming of the Old Ones and the wars to colonize the planet from the PoV of the Slaan that has more weight to it because the magic toads were there and they most certainly did not tell anyone about it so it has to be an eyewitness account.
Well, they could have told it to the elves pre-Coming of Chaos and it's recorded by their scholars researching the prehistory of the world.
The elves also have access to magic to allow for retrocognition, so may be able to investigate such things with high magic doing a better controlled version.
Also, according to one dragon in Bestiary of the Old World, he was there when they arrived and collaborated with the Old Ones to help build their gates. As there may be living dragons on Ulthuan who witnessed the arrival of the Old Ones and participated in subsequent events, they may simply have told the high elves what happened. The history in Realms of Sorcery is probably what the Colleges believe, probably based on elven knowlege.
I think it's a bit of a cop-out, really.
And it's not like they're consistent, the history section in Children of the Horned Rat straight-up says "this is not information that could possibly exist in-universe" on page 27.
I don't really see it as a cop out. It's the same thing as just about all real historical knowledge pre-photography is subject to.
And what page 27 of the Children of the Horned Rat says is quite possibly the opposite, I think. It says:
The history provided below, therefore, cannot be attributable to any inhabitant of the Old World, and should be treated accordingly.
This is a health warning saying that no one in the Old World should have been able to write the following section, so you should wonder who
could write it. Have the High Elves on Ulthuan secretly been compiling a history of the skaven to work out how to better combat them? Is, perhaps, Clan Eshin an exception to the general rule about Skaven not caring, and then recording it in their strongholds in the East. Or, once again in the east, does the Dragon Emperor command his civil service to maintain records about one of his many foes. Closer to home, but still outside the Old World, this is the kind of thing that the Lahmian Sisterhood in the Silver Pinnacle may well have compiled.
What that sentence means to me is that the history may have errors or omissions or biases because it was not written by an Old World historian, but instead someone from elsewhere with their own perspective and sources of knowledge.
It noticeably does not say 'this is out of character information that no one in the setting knows'. It says that this history isn't
attributable to anyone in one part of the setting. That word, attributable, also keeps open the possibility that people in the Old World do know this, but no one else knows that they do. For example, it take my examples above, the Cathayan or Ulthuani Ambassadors, or elder Lahmians may know this, but it's not known that they know. Also, only the skaven and humans are mentioned in the preceding passages. The possibility of the dwarves having records is ignored.
Also, the history is, in many ways clearly not the writings of an impartial observer. It says things like 'Humans and Dwarfs worked and lived in perfect harmony'. No one ever lives together in perfect harmony. Also, it says 'Their building techniques and engineering skills were the greatest ever seen on the surface of the Old World' That's very likely bullshit as well, given that there were joint elf-dwarf settlements built in their mutual Golden Age, some of which apparently produced wonders.
It reads like something an inhabitant of the city would say when waxing lyrical about how great the place was - and it could of been, given that the Lahmians would presumably have been interested in the place and may have made one of the city's daughter part of their Sisterhood, for example.