Xinzi was correct. Tieshan was never part of the Sovereign Moon Goddess and Xue Lin's plans. But her patron, the Heart-Wrenching Demon Noble, had long-noticed Xue Lin's reincarnation pattern and needed someone competent enough to not only foil her plan but also refine her new soul.
You see, the Heart-Wrenching Demon Noble is remarkably ambitious. So ambitious in fact that he doesn't mind butting heads with Gods or Devils if it means it can harvest massive benefits. Xue Lin's soul is benefit, enough benefit for the Heart-Wrenching Noble's chosen to see their Soul Force, Soul and Yin quality rise by stratospheric proportions.
This in return would make that chosen a far more productive dual cultivation partner for the Heart-Wrenching Demon Noble, and help him break the last shackles that his technique imposed upon him.