Shapiro seems to assume an economic model in which ownership is expressed through marketable shares, the economy tends to be organized around large market areas with mostly anonymous economic transactions occurring mainly through the cash nexus, etc.
And he explicitly assumes (point three) that current firm size and market structure represents economies of scale that are inherent in production technology.
All the secondary assumptions he makes about the kinds of specialized knowledge a boss must have about consumer demand and the marketplace, it seems, reflect the primary assumptions above about the continuity of the hypothetical economy with the conditions of the one we live in.
None of these assumptions is warranted, in my opinion.
left-libertarians’ fundamental area of disagreement with Shapiro and Horwitz is that our model of freed markets isn’t a slightly tweaked, somewhat more leftish variant on the existing model of corporate capitalism. It implies a revolution in the basic structure of our economy.
Relatedly, be sure to take a look at this relevant debate between Peter Klein and Roderick Long from back in 2008.
UPDATE: Another response here.