I’m sorry, but yes, the Rejects know much more than they should in-universe, and no, they shouldn’t know certain details even as members of the Inquisition.
I know full well that the Imperium is vast and certain planets and individuals are more knowledgeable than others, but this does not excuse the fact, for example, that the Rejects sometimes make references to events of the Horus Heresy that even full-fledged Inquisitors living on Holy Terra ignore (see Vaults of Terra: The Carrion Throne) and, even worse, that they sometimes talk about the Golden Throne referring to the fact that it is a machine and that it is malfunctioning, details that should only be known to the High Lords of Terra and very, very few others (see Vaults of Terra: The Dark City).
Furthermore, they are perfectly informed of the various internal factions of Chaos and are familiar with at least two xeno races (the Aeldari and the T’au).
In short, the Rejects know a lot of things on the most disparate topics, notions that, taken individually, are the prerogative of a select few: let alone collectively.
Let’s be clear: I love the different personalities they gave to each of the four Classes, but the authors made an amateur mistake by forgetting that what we, as readers and enthusiasts of Warhammer 40K, know is very different (and much more!) from what the characters who live within that universe – a universe where ignorance reigns supreme and is even considered blessed – should know. We have the Internet and the Lexicanum, not them.
A friend of mine suggested the idea that this hogwash is actually intentional and is a way to introduce new players to the Warhammer 40K universe. Maybe, but the fact remains that if I had read certain nonsense without knowing the source, I would have mistaken it for poorly written fanfiction.
(Forgive my angry tone, but this is one of the “minor” problems with Darktide that bothered me, as a long-time Warhammer 40K enthusiast, the most; but since there’s no shortage of problems in Darktide, we might as well ignore it and move on.)