I doubt techniques that suppress the interaction between the corrupted Chi and "normal" Chi are really that rare. Hardly OG Taio developed it in his years as a elite Corrupt Cultivator, after all the amount of taint would already be enough to turn him into a fiend.
Also corrupted Chi isn't only gained through "demonic" methods, as Scholar Taio explained, just the contact with the taint already gives corruption, such as Spirits and the dead. Or ambient Chi overall(Such as New Yomi though this is their whole deal).