The actual opposing argument is that it's impossible to create a billion dollar enterprise without a group effort, and for one person to end up with a billion dollars necessarily means that they made decisions within that enterprise that resulted in a lopsided allocation of resources at the end.
Period. That's it, and it's inarguable.
Every single aspect of the system is arbitrary and is a policy decision made by society. The basic building block, the limited liability joint stock company as a legal concept with some form of independent rights and entity status is arbitrary. Every lever, every part of the system, is created by people making decisions about how society is organized.
The people he is arguing with are basically saying "we want the system structured differently because this one is producing too much concentrated wealth." That's a political choice and an eminently reasonable one.
So if it's that simple, why would he feel a need to straw man instead of just addressing the actual argument? Well because he'd lose. The reality is is most people agree with this assessment of society and want it to change.
And by the way the question of how resources get split between labor and capital is the oldest and most central political problem in human history. To adopt a condescending tone while pretending to be ignorant of stuff you learn in the first couple weeks of any real study of politics or history, betrays the deception inherent in his essay.