DMT, particularly at the lower end of the intensity spectrum, plays off of those phosphene light afterimages in your vision. When you rub your eyes, you are stimulating these phosphenes. I don't think it has as much to do with DMT being processed by the retinas as much as it is a modulation to the experience you can create by doing so. There are many ways to modulate the experience, from breathing to concentration to changes in posture, lighting, sound and so on.
"Science without religion is lame. Religion without science is blind" - Albert Einstein
"The Mighty One appears, the horizon shines. Atum appears on the smell of his censing, the Sunshine- god has risen in the sky, the Mansion of the pyramidion is in joy and all its inmates are assembled, a voice calls out within the shrine, shouting reverberates around the Netherworld." - Egyptian Book of the Dead
"Man fears time, but time fears the Pyramids" - 9th century Arab proverb