I guess he mean that written form is often thought as coming from a few érudits through an intensely conscious endeavor, while oral language is evolving mostly

I guess he mean that written form is often thought as coming from a few érudits through an intensely conscious endeavor, while oral language is evolving mostly in the wild and result mostly from internalized unconscious conventions. I mean, these views can be debated, but are probably what was referred to here.