I mean, this is just proving him capable of violating or disrupting some aspects of Ronova's established order or authority. Like it doesn't automatically prove immunity or resistance to every ability Ronova possesses, especially her Death Manip since her death manip can bypass even those who have immunity. So I guess this argument doesn't work at all.
Shown absolutely nowhere.
Immortality and resistance to death manip are separate concepts. A character can possess immortality and still be vulnerable to conceptual or supernatural death effects. In Ronova's case, her authority over death has already demonstrated the ability to affect entities regardless of conventional biological life or even something without having the concept of death itself.
At best, it implies that Rerir can affect, challenge, or destabilize established laws or systems. To conclude that he can resist Ronova's death manip specifically would require direct evidence that he resisted her exact powers.
I mean, as if Phanes and the Shades can't do all that.