I thought mm had said the fire would be choked of oxygen and would go out unless fed with thermite.
None of us really understands what MM is hammering on about really, but the best I can make of it is that it's just the steady fall and ignition of thermitic dust that drives up the temperature of some 'pocket' to the melting point of steel.
How this dust survives passage through the zone which is way above its ignition point but way below 1500°C is yet another mystery that MM will fail to explain.