The mirrors don’t decrease the throw distance. They allow you to decrease the size of your display by allowing you to split the throw distance between the projector a mirror and the surface. The distance from the projector to the mirror to the surface has to be the same throw distance as would be required from the projector to the wall if you pointed the projector directly at the wall.
For example if the throw distance of the projector is normally 5 feet you can point the projector at a mirror 2.5 feet away and have the image reflect another 2.5 feet back to the display surface for a total of 5 feet.
Or so I am told.