A sump is an excellent addition to a marine system as it is of great benefit to water quality. The sump may hold additional filtration. It also adds additional gallonage to the system.
The usual safeguards have to be taken when considering anything for the marine system. One of these is to ensure that the construction material is seawater safe, in other words nothing will be introduced into the seawater by its use. The idea is to enhance seawater quality not reduce it.
There isn’t a reason why a sump has to be an aquarium. It could be a container made of plastic or anything else. However, is the material in question safe? Metals are out, even if they are coated in some seawater resistant covering. Plastic that is intended for food or aquarium use is safe.
The thing is, what of the shape? Can a safe container be found of the correct size and shape to fit the available space for a sump? There are containers available, but they are usually too small and impractical. Safe plastic buckets are available (such as used for brewing) and these are sometimes used with automated top-up systems. They’re not really suitable for a sump though.
So there’s the aquarium. They are known to be seawater safe. The area can be divided up easily if required. They come in all sorts of sizes, from small to very large. There is going to be one suitable.
So a sump doesn’t have to be an aquarium. For practical reasons though, why should anything else be used?