I think the idea is to mix up a solution that is safe to handle and pour into the tank. Then add water to the tank as needed.
You could do a lot of damage to fiberglass, gelcoat, fuel tank hoses, clothes, yourself with undiluted bleach. That's why all the instructions I've seen always start with diluting it before you do anything else with it.
You could mix up one five gallon batch at the intermediate dilution level, put it in the tank, then add the rest of the water. Or you could add ten 5-gallon batches at the final dilution level.