Why is that the smallest possible unit? It may not be especially practical, but it's certainly possible to break Amazon up by retail product category (one company for books, one for kitchenware, etc).
Which employees go where, who gets what IP, what happens to all the underlying website/physical infrastructure? A "3rd party marketplace" unit is already likely smaller than is feasible for a court to oversee. By comparison, it would be a lot easier to split Amazon into "AWS" and "not-AWS" because the company is already organized that way.