It wasn't like Lex hadn't activated his lingering phoenix affinity at all. Even without excessive contact with phoenixes and any associated treasures or materials, Lex's body inherently demonstrated some similar abilities.
He was always excellent at healing - especially others - to the point where his blood was literally better than most healing medications in existence. The only reason he wasn't extremely proficient in healing himself wasn't because he himself lacked healing capabilities, but because his body itself was so powerful that healing it required a lot more energy and resources.
But being good at healing, and being able to be reborn from an extremely wounded state were two entirely different concepts, and were not even on the same level.
In Lex's current state, exposure to Dao radiation had brought him on the verge of death. The reason it was on the verge, and not actual death, was because of his 51% Dao Body giving him some minor resistance to it.
