If you have a baseline amount of protein say 100g your body will use all of that protein to produce ATP given it has excess calories (Carbs, sugars) to do so. Mass gainers are usually a mix of whey protein and complex carbs to ensure you have enough carbs and protein to repair muscle.
The prisoners wanted the milk (whey) or tuna for protein and needed the complex carbs from instant noodles or bread. Their body had enough energy to convert the protein to ATP because they had the excess carbs to do so.