I would say every inch of a plants surface can produce thrichromes, and ALL the THC is contained inside of them. Sun leaves and stems have small, occasional thrichromes which arent as concentrated but through filtering, amount to enough to produce effects. I know my plants sun leaves and stems had them after only a few weeks of flowering. Ive heard of them appearing on the first set of leaves even.
UVB light kills plants, which is why plants couldnt survive out of water until the ozone layer was formed. I figure plants need access to all visible wavelengths and some infared/UVB, which all contain forms of energy. The lower the spectrum though, as with UBV, the more energy. HPS lighting doesnt produce any blue/UVB light so I think plant health would increase no matter what given the right spectrum. So of course the resin production will be increased. Its too bad you cant see what is actually going on in the plant.
He says to use 270-300nm, I wonder if they make LEDs on that wavelength? What type of day/season has the most UVB output? I do believe his bit about how all the best plants come from areas with high UVB exposure.