What matters is the frontal area of the parabaloid, assuming the LED is in focus, so when looking into the reflector you see it filled up with the image of yellow phosphor. The flat reflector you mention would not focus the light from the LED so you would not count it as effective area.
Consider just a flat domeless LED. It has a fixed area, say 2mm^2. Now imagine looking at it from the side so you only see a very small sliver of brightness. The light intensity at that viewing point is very low because the area of the LED apparent from that perspective is very low. The same concept applies with the reflector; just the apparent area matters.
Also the numbers you’re using for the diameters for the M2/M1 reflector are the outer diameters I think, not the diameter of the reflecting area. See here for measurements.