Another point that was not very clear the cure cannot cure the sirens, and the sirens are the only immortals that could be half witches, because their immortality is a kind of improvement in the user who owns it,
Cade prevented the soul, from leaving your bodies to hell or another dimension, it would be like a kind of armor in the user, because you are still a witch or a vampire when Cade gives you immortality, Sybil said you have to be supernatural, moreover it’s not genetic immortality like Silas’s because sirens only have the hardness of their bodies and their healing factor increased, so the cure cannot cure the siren's immortality because they depend exclusively on Cade for that.