Just because you don't think it should doesn't make it so. Necrotic means decaying...which means it is blessed by Nurgle. Just because the devs forget to make it restrictive doesn't make it unaffiliated. Why else would it be Poisonous with Deathguard colors and "filled with a spirit of entropy"? Are you also gonna imply that the plasma pistol that tells you in the description that it is Slaaneshi-based isn't just because other Marks can use it?