But how? The smallest unit of a possible breakup would be a standalone 3rd-party marketplace company, and yet that unit would still have the same market power for 3rd party sellers as Amazon as a whole does today.
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.