I think your all confusing 'flat shading' with good lighting. The thing that stands out the best from all of the flat surface models, particularly this
is the lighting. Beautiful GI lighting. The reason it looks so good with 'flat' single colour surfaces, is that the lighting information isnt hidden with textures, It creates nice 'soft' surfaces. The two (good GI and flat surfaces) complement each other very well.
'Ambient Rendering' is a technique used by environment/character artists, to purely show geometric detail. The crudest technique is to apply an AO shader/light in the scene. Better examples use full GI (Environment/Character artist, usually dont know how to light jack, so they use the crude GI method).
Good exmaple of a GI render
Ambient render