From the info provided by GG, I would say only the first frame isn't 1920x1080 and the ones coming after are, interpolating the missing data with the previous ones. It's a nifty way of achieving a native frame buffer to be sent to the TV/monitor.
I think where people are hung up is on the concepts of native and scaled (up or down),
The usual pipeline to get an image to the a monitor is:
Game engine computing > FrameBuffer(s) > Frame > Monitor Buffer > Monitor Image (the image you see on the screen)
With the advent of fixed resolution monitors (LCDs and the alikes) there was a need to have a new step/component to get a properly formatted frame on the screen if the frame coming up isn't of the same resolution of the monitor, a scaler, so the pipeline this days translates to:
Game Engine computing > FrameBuffer(s) > Frame > Scaler (if it's being handled by the machines hardware) > Monitor Buffer > Scaler (if it's handled by the monitors hardware, usually a frame always passes through this one, if it's native it's a simple pass through, if not it's processed accordingly) > Monitor Image (the image you see on the screen)
To put it very simplified all the visual information computed by the machine is always stored in a framebuffer that is then translated to a frame so it can be sent through the normal pipeline. This information can be used in many different ways and the framebuffer isn't a fixed size or number (you can have multiple frame buffers for different things to create the final frame).
Usually a frame (created from framebuffer data) is always created in a fixed resolution, like 1280x720 or 1920x1080 and when it hits a scaler is treated accordingly: if it's native to the screen resolution it passes through untouched, if not, it's scaled to the screen resolution.
What I gather from the GG explanation is that they reuse frame data to create missing data and patch the gaps that are not being created by the usual game engine rendering pipeline (meaning that in a 1920x1080 frame, half of the horizontal resolution is being 'guessed' based on information in the previous frame). Meaning the actual frame being sent to the monitor is still native (meaning 1:1 mapping to the screen resolution), apart from the first one. This means there's no up scale of the frame and therefor it can be considered native.