Though the motivation is good, this seems like a fairly round-about way to introduce boxes. It sort of claims that boxes exist to create types that are pointer sized, which is a fairly novel way to think about boxes. It also is a bit of a tautology, paraphrasing "boxes use allocation to be the size of a pointer", since "box" and "pointer" are essentially synonyms, this is saying "boxes use allocation and are the size of boxes".