The weight savings comes from the material it's made of. From their website:
"Cargoshell is manufactured from composites and therefore much lighter than a steel container."
"Cargoshell is manufactured from composites and therefore much lighter than a steel container."
why not make collapsible metal container? (if it's needed at all)
So you are likely to have a container that costs more, is heavier, isn't waterproof or as secure and is more expensive to handle. It's only benefit is lower volume for the return leg - volume isn't a big problem for ships.
It's like building a convertible - it always ends up heavier more expensive and less stiff than a regular car.