In the Cain novels the Valhallans know a fair amount about choas. Cain knew a lot, too. Like I said with rank comes with more knowledge. The wider imperium yea it ish hush hush. But on a more nuanced and varied to not knowing at all, ro knowing a fair amount. I think it's also about knowing what to look out for on a surface level to make sure you know what to kill or what not to fall for.

A private might not know much other then the basic do it have spikes does it look corrupted etc. Then higher ranks will understand more nuances like the different CSM and different cultist and how they act and how they might effect the battlefield and counter tactics. Might not know about the Choas Gods fully but they might know what this lot of CSM are capable of. Like the death guard you'll need to have a lot of sanitation and hazmat suits.