At high temperature, a bare metal surface exposed to air simply combines with oxygen (and sometimes sulfur) straight away, forming a scale on top of the metal, this chemical combination is oxidation. Diffusion of gas or metal ions through that scale controls how fast the scale keeps growing, but the reaction that actually eats into the metal is oxidation, not diffusion by itself. Mechanical wear and plastic deformation are physical damage mechanisms that have nothing to do with a chemical attack from the surrounding gas. So high temperature corrosion is fundamentally an oxidation process.