Parent post is not arguing about underpaying, but the feasibility of increasing wages. The idea is that if we treat all companies and their employees as one grand company, then the company will have to sell everything to its employees in order to get revenue, but that revenue will have to be distributed to the employees as wages, in full, in order for everything to be bought. That leaves no margin for profit.
The answer to the question lies in what is missing from the equation: the existence of stockholders. They are potential customers too. You don't need to sell everything to just employees, that means you can underpay them and still be sustainable as long as stockholders buy things too.
But there are limits to how many things a customer wants to buy. Once the stockholders' needs are satiated, the rest of the profit won't be spent. At that point, in order for businesses to be sustainable, this balance has to be redistributed as increase in wages.
The answer to the question lies in what is missing from the equation: the existence of stockholders. They are potential customers too. You don't need to sell everything to just employees, that means you can underpay them and still be sustainable as long as stockholders buy things too.
But there are limits to how many things a customer wants to buy. Once the stockholders' needs are satiated, the rest of the profit won't be spent. At that point, in order for businesses to be sustainable, this balance has to be redistributed as increase in wages.