You need a small amount of insulin even during ketocis, otherwise you will go into ketoacidosis. Type 1 diabetics suffer from destruction of b islet cells and thus have reduced to zero insulin production. There are some reasons why you might have a small insulin production( e.g. acute pancreatitis destroying some but not all b islet cells) but those are pretty rare. For most people the destruction happens as a result of an auto-immune disorder and eventually they all get destroyed. So maybe there's a period between the diagnosis( usually around ~90% destroyed cells) and when all cells get destroyed that you can manage it with keto. But it won't last forever and thus is not a cure.