Open settings, turn on 'invert colours' and go to a pitch black room. In the settings app with invert colours on you will have a dark screen but you will see it's still emitting light- particularly noticeable if you hold your phone at an angle, where you may see a bluish hue which is not there when you turn off your screen. This is present even with brightness turned all the way down.
With invert colours still on, turn reduce white point on and off. You will see there is no change in the black or the amount of light coming from the screen- the white text is just a bit dimmer. The light coming from screen does not change in the same way as turning brightness up or down.