As for "Soul-Refining True Technique", Luo Chen had not cultivated it.
After all, it's merely an incomplete cultivation technique, and if he tried to cultivate it, he would exist in a state between human and ghost.
Even if he managed to break through to the Foundation Establishment Stage with its help, he would still suffer from the after-effects of the fusion with a thousand souls, which would be an endless source of trouble.
However, these problems were non-issues for Bai Meiling, who was of the Yin Soul disposition.
She was already a Yin Soul, and there was no question of being neither human nor ghost.
Moreover, in the future, when she completed the Ghost Soldier Realm Finalization, she could attempt to establish her foundation using the methods in "Soul-Refining True Technique".
After-effects?
Not a problem.
Bai Meiling's symbiotic entity was a Ghost Mill, capable of crushing ghostly creatures.