Yoshiro hasn't become the spirit stone yet when Soh purifies her.
Because Soh is the final part of her that becomes defiled in order for her to become the spirit stone, hence why Cursed Soh is definitely previous Yoshiro. The two are one being playing two roles.
Possible Correction:
She hasn't because she hasn't absorbed the final defilement which is Soh's Spirit Stone. That completes it.