Both game default resolution & the export (as far as I'm aware) resolution options only affect the default resolution of the game, they don't automatically scale down the graphics to the new resolution
No the graphics are not really scaled down (the pictures inside the game files are still in 1920), but the result is different in both cases. Here's what happens:
Image1 (top): normal game resolution (1920)
Image2: low resolution changed with build
Image3: low resolution changed with game
What happens in image 2 is exactly what I want. Even if the images are not really scaled down, they appear to be. I think that the whole graphics are stretched to the 720 resolution, and then upscaled to fit the desktop resolution, which gives the downgrade aspect that I was looking for.
Unfortunately there are too many glitches/artefacts in this mode and it cannot be used properly. It is not visible in this picture but there are a lot of strange things on the screen with this option.
In image 3, the game resolution was changed to 720 but the images are not scaled to the screen, which results in a wrong size of the pictures. Everything that is more than 720 pixels wide is automatically cropped and the rest is upscaled to fit the screen.
Anyway that was just to explain what happens, in case other people are curious.