The underlying structure is not the issue, it's the object literal that doesn't have its key order defined (in older standards) and therefore the interface as suggested would be broken.
In practice the order will be maintained in the major engines, but that's still not a good reason to do rely on it.
I think the parent is suggesting taking a map instead of an object as the interface for constructing the Struct objects. Not using it as the underlying structure.
In practice the order will be maintained in the major engines, but that's still not a good reason to do rely on it.