That sums it up quite nicely imo. Sexual expression of cannabis is a phenotype (
http://link.springer.com/article/10.1007/s10681-004-4758-7 ) and as with most phenotypes, their expression is influenced by the environment.
No (unless it was done for many many seed generations). And yes for the second part, cannabis plants that don't herm (show intersex traits) aren't necessarily not prone to herm but may just have high stress resistance.
Even though they said they never have problems, they may simply not notice them (it delays flowering above all and even if no intersex traits show it doesn't mean the plant is running as optimally as it could with a proper photoperiod).
I think it's in "Life, the science of biology" (fun book if you like me didn't pay attention back in school... "because I got high"...) they show a test in which just part of the plant was covered in darkness and it still builds up enough flower hormones for the plant to switch to flowering. Point is, in a setup with 20 small plants with a light leak shining on a single plant in the corner it may more likely cause intersex traits or reveg than for example 4 plants on the same space with only 1 bud site/cola being affected by the light leak. This in turn may contribute to the different experiences people have with light leaks as well.
I have a cross (Late Night) that I suspect might have a selfed grandparent and while I've only grown out two so far, I've moved them in and out my closet, from dark time into daylight and back, as in really messing up the photoperiod over and over, and I just cannot get it to hermie. As they say, ymmv.