That doesn't make sense to me, the article clearly says "pre-rendered". Maybe they don't render the middle of the ocean tiles beforehand but there would be no reason not to save a tile once it has been rendered.
Article says that cells sent to device prerendered it doesn't say ALL cells prerendered.
Obvoiusly they won't re-render tiles that have no new information since last access, not to mention middle of an ocean or desert.
Right, but there are layers that can't be pre-rendered (satellite) or are probably slow to render (terrain).