Look at my diagram. On my screen the focus distance is original 5cm, the distance d is 0.6 cm. If I expand that first focus distance to 5m, so 100x as far, than the size of d is going to be 60cm. Based on measurements from my screen and on that specific aov and angle of rotation.
Once you know that you can calculate the dof belonging to the wanted diaphragm and make your conclusions. The dof is divided in a part before and after the subject plane.

It seem to be common here to ridicule the author rather then trying to understand what he wrote.

George