Yeah I researched it for a couple of months and found tons and tons of research that all says the same thing - the water constant is miniscule in comparison to the water constant.
While the reflection from tank walls and plants change the refraction of light in the water, you will lose more light in those 2 inches of air space above your tank than in the entire depth of your tank.
In the wild, light penetrates at different levels, the first big fall being the 6700 spectrum at 15 feet. The next is at 25 or something like that, I didn't focus on deeper depths. We obviously have plant life deeper than 15 feet because different plants have different needs, ie salt water plants and animals thrive under actinic lights because that is one of the spectrums that penetrates to the deeper levels. So unless your tank is deeper than 15 feet, you're good for growing freshwater plants lol.
Somewhere on here or another forum I posted some awesome research on it. There were some really cool studies where they measured the diffusion of different types of light at different depths and found them negligible. They also lowered and raised the lights above different tanks and found that it makes a HUGE difference. I'll try to dig them up or you could try goggling it.
Are you sure that's accurate? I would expect it to be the opposite. Consider that sunlight penetrates over 600 miles of Earth's atmosphere to support terrestrial life, but once that light hits water, it supports abundant plant life under maybe the first 15 or 20 feet feet of water.