Light field displays literally show a different image to one part of your retina than another. It is not incorrect to explain that technologies like the rift are 'flat', as, while they may show two images, they are simply flat images on a display. Light field tech combines an array of images that recreates the way light works in reality.
Light field displays are to the Rift like the Rift is to a 3DTV.
The Rift and 3D TVs are fundamentally the same. Light field adds 2 dimensions of information (2 axis that control the angle light travels). To me it is the difference between (2D+2D) and (2D×2D), or <x,y,z> and <x,y,z,∆x,∆y,∆z>.