Incorrect in the big rule book on page 22 in the black box with red borders bottom part chaos psychic focus clearly saids that for this exsample if hes a daemon of nurgle because hes a daemon prince of nurgle he automaticly gets the primaris power for nurgle abiltys for free and no he does not need to roll one as it clearly states he must select at least one power from nurgle discipline in the codex for chaos space marines so having the primaris power automatically means you have fulfilled the requirement's.