Agreed. Zhang Wuji needed to learn 9Yang (for powerful pure Yang energy) in order to get rid of the remnants of the poison which had spread to the three vital areas. But I want to point out something here.
Initially, ZSF and his disciples thought that they had cured ZWJ of the poison. However, 36 days later, ZWJ became sickly again and in grave danger. An error or misjudgement from ZSF apparently?
I have the impression that if it was (for example) Shi Potian who was healing ZWJ initially, instead of ZSF and his disciples - Then ZWJ might have been cured earlier.