The article says the estimate was based on the erosion rate of the rock dolerite. It does not say who made the estimate, what dolerite was measured, or if any other kinds of measurement were proposed or tried. Personally, I am interested in the widely spaced extensive “irrigation ditches” covering an area more than 300 miles square west and south of the lower Okavanga delta. I tried to find photos that previously existed at Google, but did not succeed. Look for them on Google Earth. Large number of parallel lines spaced about 1 mile apart, over large area. ridiculed as parallel sand dunes, but I think they are too regular, why being hidden from study?
They have been studied, and they are 100% natural.