If you look at the linear programming graph in the answer, you will see that the optimal solution is at point D (explained on the previous page of the answer), and that point D lies on the craftsmen line and the max demand for S line. This means that it is using all the craftsmen time and that it is producing for max demand.
However the point D lies below the ash line. That means that we are not using all the ash that is available and that therefore ash is not critical.