Even the article you are citing does not support your statement.
Vitamin C is very much storable in the long term. It's just that the raw form of most of the natural foods that contain vitamin C are not a suitable medium in which to do this storage in the long term.
Yes, it does state that "Vitamin C, for one, breaks down easily in foods" in "some astronaut food staples". It is a matter that needs some attention, meaning they need to provide supplements or select some food forms where the vitamin C (and vitamin B12, and folic acid, and vitamin A) last longer.
However there are food types and storage methods that do preserve these quite well.
For vitamin C, it is the presence of oxygen and water and/or heat in the stored food that degrades the vitamin C. A suitably dehydrated product, or one where the food is stored in oil, will retain its vitamin C indefinitely. (Well, longer than your 5-year requirement). Vitamin C stored as a supplement of anhydrous ascorbic acid powder and stored in a freezer will last, literally, centuries.
Obviously fresh produce is preferred where at all possible, and palatable forms of stored food are needed, and the final nutrient balance of the food must be correct, but the situation is a matter of optimizing the best method, not an insurmountable obstacle.