Armor Box/Satchel Recipes
I think you misunderstand something.
You can just make the other pieces of the armor set that you like.
If you use the Mystic Forge with the chest piece, then you will be given a recipe, not an armor set. You would need to learn the recipe and use crafting materials to make the rest of the set.
The bug that the wiki is referring to is, specifically, that the box set recipe takes lots more materials than the individual items summed together. The recipe is supposed to take approximately the same amount of materials as crafting a full set individually. I haven’t heard anything about it getting fixed yet.
It would never make sense to use a chest piece to make the box set recipe for a single, personal set of armor. The box sets are meant for people who intend to mass-produce armor, for sale or for their guild mates and friends. You would always be spending more to make the box set once than to make the individual items once – because you have to “buy” the recipe from the forge first.