The skill description for Psyniscience does not mention it requiring a Psy rating to use, and it uses perception rather than willpower to function, so it could be mistaken by some (such as myself), to be a skill anyone could use.

However the Psyker trait mentions you could take Psyniscience now if you have it, etcetera.

Therefore it would make sense if the entry were corrected to state that it requires a Psy rating or the Psyker trait to take the skill, would it not?

It does, however it is only stated in the description of the "Psyker" trait (p 142). It's a bad case of copy-paste since in previous systems only psyker-like characters could purchase psyniscience.

The only exception I see is if you gain psyniscience from gift of the gods.

That is what I thought.

In the RT line, non-psykers native to the Koronus expanse (termed Footfallen) all had the Psyniscience skill. It's not hard to imagine such oddities existing in the Screaming Vortex as well.

In another oddity, in DW's Rites of Battle, one of the Solo Mode abilities available when randomly generating a Space Marine Chapter allows a Battle-Brother, regardless if he is a Librarian, to gain Psyniscience as a Trained Skill at character creation. The chapter, apparently, has been carefully trained to detect disruptions in the For-…I mean the empyrean.