Its a concept thats been around for ages really. Flat shading is actually a lighting model, a very simplistic one. Rather than smooth out the lighting on a face so that it can be light at one corner and dark at another as it goes into the shadow or something or whatever, the entire face is lit uniformly based on the angle between the normal vector of the face (the direction it is facing) and the direction of the light source. Here are some picture examples...
FLAT SHADED
GOURAUD SHADED
Obviously in HL mapping, you cannot have control over lighting in order to achieve this (well, you can fake it through using exceedingly high texture scales or chop values) but a similar effect be be reached by using only flat colours and not letting them be effected by lighting.
Its certainly not a concept I invented, nor any HL mapper invented - its a lighting model and concept that has been around for a long time.