John McPhee's book "The Control of Nature" is based on sound science. One situation that he investigates is southern California's policy on allowing residential development in areas where natural fires are quickly extinguished and terrain is geologically unstable. The result is chaparral build up. Chaparral is like tinder during dry summer months and when fires flare up the roots binding unstable soil are destroyed. Winter rains wash away the unstable material resuling in mass wastage and damage to man-made structures.
Fires do not "harden" soil except possibly in places where caliche or other cementing agents are naturally present. In most fired areas soil is nourished by organic material whose breakdown is accelerated by fire. This results in more permeable and fertile soils.