The author’s entire thesis is that there isn’t a market for lots of goods because of oligopolies colluding. Supply and demand don’t work like the textbook says they will if there’s no market. He’s saying that almost all potatoes are sold by a couple firms, and those firms collude on price, effectively meaning (from a pricing perspective) that there is only one potato company. They therefore can charge whatever they want, up to the point of driving their customers out of business.

This is in contrast to a healthy market, in which producers compete by lowering prices to the point where the producer would go out of business.