I believe IcyFox has once suggested that to get to the peak of internal, there is 3 ways to do it.
1. Mastery and fusing both 9 Yin and 9 Yang
2. Mastery of Dragon Elephant Prana Skill Level 13
3. Fully Mastery of the Damo Yi Jin Jing
And as for the purity, I pretty much agree with Chronoreverse's explanation. His internal was not only inferior in terms of purity to Z3F but also the 3 Divine Du Monks.