I would completely disagree with that. If you look at chaos focus its seperate from the psychic focus rule. I would push forth the concept that you are given the primaris or as some have said a d6 power role for your specific god which would then fill the requirement that you need to take a power from the CSM book. when you look at how they are selected they do not have an order for when chaos focus comes into play. there is no step by step order of operations. therefore taking that power as a free roll still allows you to roll for you mastery levels. thus taking say typhus you would end up with your primaris or free roll then 2 additional rolls along with force for a total of 4 powers for your level 2 psycher.
From what i have seen and seen it ruled as your not able to have psychic focus for a particular discipline via the chaos CSM codex as its currently written thus to balance it out you have chaos focus which means your not penalized for taking a marked sorceer. other wise if you look at a marker sorceer then they completely fail when compared to psychers from other armies as well as unmarked sorceer's. chaos focus is just a balancing tools. other wise as you would want it played the CSM marked sorceer would only ever have 3 powers force being one of them and they are penalized for taking said marks. if you look at the intend its all there. other wise chaos focus makes no sense what so ever. yet when you look at is as being the tool to fufill the codex requirement it makes is so that you do not loose the benefit of being psychically focused in a particular discipline.