Yep, it all that other stuff, item A, B and C above that grows plants and which are responsible for patterns and so forth. Nothing to do with intensity. It just seems to us that intensity is a prerequisite because when we do apply more CO2 and fertz and circulation then the prize that we are looking for happens sooner rather than later. This is an illusion due to our lack of patience.
The worst victims of this illusion are carpet plants like HC, which require lots of CO2, nutrients and flow. But everybody thinks they need more light, so they pump gigawatts of energy into the tank when then requires more CO2+Flow. You can get into a never ending cycle of more light then more CO2. This is why you'll see where some will say this plant only grew well when they added tons of light plus CO2. But this is a self-inflicted syndrome, first because the hobbyist expects that more light is needed, and secondly because HC is slow to adapt to submersion and is a generally slow grower anyway. Once it adapts and gets going, under high light and high CO2 it will pearl like crazy and huge mats of it will actually lift off the substrate just from the buoyancy of all those O2 bubbles.
Under a lower lighting regime, but still with good flow +CO2 + nutrients, HC will fill in later than most would like, but the problem here is that HC is so expensive and is sold in such small portions. Since most can only afford, or can justify spending the money for only a few measly pots it becomes imperative to have accelerated expansion, otherwise the carpet looks threadbare for a long time. This only serves to reinforce the myth that it "needs" high light.
Cheers,