Responses
Yeah maybe there's an exemption you know stories like this always have twists and turns exempting some rules. Either he'll be a vampire or turn to his link.
Example of good twist with exemption is K's secret. Technically it was implied there that the only way for Doyun to survive is if was turned to a pure blood vampire through Nux's blood but that would mean that he can't be with the director anymore. But who would have thought that the Director can ingest the blood of Nux turning him to a food supply for Doyun. Well it's just my sample of exemption hehehe.
This is just a theory what if Sooin can become the blood link of the Seme. I mean there's actually a way for Sooin to continue feed the Seme his blood and still become immortal and that's if he becomes the Seme's blood link. Problem solve and win win situation .