No, it can't dim the brightness, except for using the OS's built-in dimming function. Usually, the only way to dim it further than that is to solder an extra resistor into the circuit
What your app does, is just adding a tint to the screen. It can be a pink tint, it can be gray, but the brightness will remain the same. Only that the colors displayed will be darker.
Everything changes when you have an AMOLED screen (or was it OLED? Well, the one that conserves energy when pixels are black). Then you can make some of the pixels completely black, which reduces the resolution, power consumption and the amount of light that comes through.