The problem is that the intensity of the light drops proportional to the square of the distance from the LEDs. That relationship can't be defeated. But, with lots of LEDs, the light source is distributed all over the top of the tank, so as you get closer to the LEDs, the rise in intensity stops going up with the distance squared, since fewer and fewer are illuminating that spot. That means, with just a few LEDs providing all of the light, that inverse square relationship remains valid up to pretty close to the LEDs.
The point you missed is that if you have 50 mms of PAR at the substrate, with both setups - one with the LEDs 1 foot above the tank, and the other with the LEDs 2 feet above the tank - the one with the LEDs closer to the tank has a much higher PAR near the water surface than the one with the LEDs farther from the tank. But, both have 50 mms of PAR at the substrate.
It will work either way, but the higher above the tank the light is, the less variation in PAR from top to bottom of the tank. The sun is 93 million miles away, and the sunlight intensity varies not at all over even a 10 foot distance. But, our lights are typically right at the top of the tank, so the intensity varies a lot over a 2 foot distance.