Impact of Import Quota onto Revenues
There is no mistake. The solution: it is assumed that there is only one price at the domestic wheat market. Hence domestic producers will not sell at price $P_w$ and price $P_{Quota}$ as well. This makes sense: Suppose you are a domestic producer and you are aware that the price that results from the quota system is $P_{Quota}$. Knowing this you would not sell your goods for the lower $P_w$.