It's in the BRB actually, on the same page as the Psychic Focus rule.
Perfect! I was wondering, since I knew I had even read it before.
When you're level 2, things get complicated. You MUST ROLL one power, but you CAN'T roll two powers, because it says 'may roll up to half'. This disqualifies you from maxing out and getting the Primaris. That's why Chaos Focus exists; so you always have the Primaris from the Mark table as a consolation prize.
This is right on the money, it's totally a consolation prize for not being able to gain Psychic Focus normally.
Just looked up the rule again. Yes, it looks like Horrors could get away with it. Demons can skip rolling on the god's powers, but Chaos Psychic Focus still gives it to them. THAT is the loophole, since it's meant to be one Psychic Focus or the other.
Pink Horrors generate their powers from the Change table only, and as such would get normal Psychic Focus without needing Chaos Psychic Focus. How do they get to skip rolling or gain powers from another discipline?