You are correct that frostbite is typically the cause of necrotic toes and feet. But there was a fairly recent thread about a wild pigeon that lost its foot due to a rubberband wrapped around its leg, which cut off blood supply. The pigeon was very lucky that the OP of that thread caught it and gave it a good home.
There are some rare vascular disorders and syndromes that can cut off blood supply and cause necrosis in birds. Very rare, but it does happen. (In humans, diabetes is the most common disease that can cause loss of toes, feet and legs, but there are other disorders too.)
@horseloverfarmgirl originally made this post because the Silkie chick had bumblefoot. Without photos and updates along the way its hard to know what happened, but if she said her chick was never exposed to freezing weather, I have no reason to not believe her.