the point is simply that bug physiology is different than human physiology. if a bug gets a bit of borate on his self and ingests it while preening, it poisons him (neurally, iirc). but for we humans, we'd have to eat it by the spoonful just to get a bit of hormonal disruption. environmental exposure to borate is non-toxic, and we actually need a small amount.
so, there are ways to kill bugs without any significant harm to mammals. i use Dawn dishwashing detergent to kill the fleas on my cats. it is surprisingly effective.