Stacked camera sensors have been a thing for a while now; they have improved sensitivity to light by stacking translucent photodiodes on top of each other.
The Nichia 719A is the inverse; translucent LEDs are stacked on top of each other (in a single package) for increased intensity.
I don’t. I know he got a few emitters to experiment with, but it’s anybody’s guess when he gets them in quantity.