It ought to be obvious,
...that there will be a spectrum of damages done to the microbes in question. Some will be killed, some damaged "so that they cannot reproduce," and some will be damaged to a lesser extent.
Therefore, evolution will continue among the survivors, and I think it's inevitable that sooner or later some will have developed an immunity of some kind to the light.
Also, I wouldn't be too ready to conclude that humans are never harmed by such lights. The acknowledged mechanism is that the light destroys RNA inside the cell. If it does the same in human cells (and that destruction would have to be indiscriminate), then I see a clear risk of skin cancer.
The reason they feel free to make the claim that it's safe, could simply be that they haven't made that link YET.
