Well I think a plant builds up secondairy metabolites which then can absorb said radiation before it damages cellwalls or DNA of the inner plant structure. Perhaps cannabinoids are a part of this shield - at least its proven that they'll get broken down by UV radiation, and that a plant subsequently refills it, making Trichs longer.
Outside the UV delivered to a plant is not qually distributed at all. It varies greatly day by day. And there's surely more UV when the sun is at the zenith than low above the horizon. So a few strong hours indoors seems, well, at least natural, and would give a plant some more time to initiate repair or refilling processes.
I've already seen plants that were accidently wasted due to a bad controller - UV clf shined whole night - sunburnt as you've never seen, a complete tent nearly lost, bleached and dried out leaves that crumbled to dust upon touch... I've seen this outdoor, too, esp. when vegging indoors under non-UV emitting light - all leaves get bleached white, fall off, but new leaves grow back right into the same sun, and stay healthy.
I have alot of trouble with some of these scientific studies because you'll find both methodic errors - and esp. also interpretational errors from readers etc... so I personally wouldnt go so far as to make absolute sentences judging from a single study. For example, there's been this study comparison flower-weight Mh vs HPS --> outcome HPS wins hands down! because this is established because what most users look at is the result. Ofc its not without error, but in a forum where so many people from all over the world come together and share experiences, I guess there'll be some red line established in this "ground knowledge".
But coming back to the study & your post - did these studies really give both chamber species the exact same amount of potential for photosynthesis, so that this variable can be zeroed out and reasons found elsewhere. But what - a HPS puts out more photons, esp. in the 600w regime its 90k HPS lumen vs 60k CMH or 50k MH. So maybe they measured PAR out but then theres your mentioned quantum photon efficiency, has this been accounted for? And difference leaf surface and what not... then an MH has alot of different spikes... just to make an example
I would find it better for studies if LED monos are used... however, there are so many things to be considered it's basically mindblowing to try include them all... from UVB to green plants have so many different peaks in the uptake of light in certain regions, I wonder how they all interplay with each other, or what they tell a plant about its surroundings individually...
I also have a hard time understanding why a red 660nm receptor should tell a plant that it's "day" when inf act the blue light is hitting a plant even later, twice each day. Then there's this difference in canopy penetration and photon efficiency. The ones that's best absorbed is also absorbed quite swiftly, but it heat that part of the plant up but leaving nothing to the rest.... which, depending big plant/ tiny plant may just be both good depending on the situation...
Furthermore, some folks out there simply try to mimic the sun while others try to manipulate a plant with delivering special light frequencies. I also wonder if this manipulation is different from plant to plant, that is, it seems that having the ability to turn special frequencies on/out is mandatory.
And you don't really need to ask for cannabis-specific studies because many plant function the same. Yes, they mostly LOOK different
I know what 
Lol
< Joke