The reason Vincenzo's Hb is only 12 is because of the hemolysis. It is destroying his red cells prematurely, causing his bone marrow to become overactive replacing them. Even though he is able to readily create new red cells and keep close to a normal Hb level, the hemolysis is taking place. His problem isn't low Hb. It is what this hemolysis leads to in terms of overproduction of RBCs and excessive absorption of iron from the gut to feed these new cells. The iron is the real problem he faces.
From what I read, certain mutations can create functional RBCs that are in some way abnormal (size, shape or surface abnormalities) and cannot pass through the filters in the spleen and liver, and are then destroyed by those organs. For simplification, compare it to a large truck full of food for the market, but there is a tunnel the truck must pass through to get to market, but the truck can't fit through the tunnel, and gets stuck and the food ends up going bad and then needs to be replaced. Even though Vincenzo's RBcs may serve to carry oxygen throughout the body, because of some abnormality, they are removed by the spleen and liver because they don't "fit" through the filters in the organs.