This is not the case at all. The updated
BRB FaQ doesn't state in favour of either opinion (yes they get both/no they don't get both) and neither does the current Daemons
FaQ.
However the Draft
FaQ for Daemons says they get both:
Q: If a Chaos Daemon Psyker generates all of their powers from a single discipline from Warhammer 40,000: The Rules, does he benefit from Psychic Focus in addition to Chaos Psychic Focus (as the mandatory primaris power from Chaos Psychic Focus isn’t generated as such)?
A: Yes
Link