This is straight science....
Short: UV light breaks the central instruction list of cells, the DNA.
Long: All living things have DNA and RNA in them. (some viruses have only RNA, but viruses don't really 'live'.) DNA and RNA absorb ultraviolet (UV) light, with a particularly strong absorbtion band at about 286 nm (a wavelength of 286 billions of a meter). When the light is absorbed, its energy is changed into motion of the DNA molecule.
Once in a while, this motion breaks the DNA molecule. Unless the break is repaired, the DNA can't be used as a template for other molecules when the cell divides, so the cell can't produce viable daughter cells. If the break is repaired incorrectly, the daughter cells' DNA will have mistakes in it. Either way, the proteins encoded by that section of DNA can't be made, and the cell loses the functions determined by those proteins and that DNA.
Most cells have several repair mechanisms, but they often don't work. Exposure to intense UV for a long time breaks the DNA at so many places that there is more work to do than the repair mechanisms are capable of.
If the cells are in a nutrient broth, and only 1% survive, it makes little difference. Many common bacteria divide every 25 minutes. Many yeasts divide every 90 minutes. Since there are twice as many cells after every division, there will be 1000 times as many after ten divisions. (Do the arithmetic, and covince yourself 2x2x2x2x2x2x2x2x2x2=?) That's less than 5 hours for bacteria, and less than a day for yeast. So you want it to be a lot more than 99% killed.
Also, the ad doesn't say how clean the surface must be. If the bacteria are under a thin layer of dirt, the dirt could absorb almost all of the UV, protecting the bacteria and yeast. To remove the dirt, use detergent and amonia, both of which will kill almost all bacteria and yeasts anyway.
Finally, their note that UV can damage the eyes is important. Skin cancers are very common because UV light from the sun (which probably puts out 100 times as much as the light for sale) can penetrate into the skin. The base layer of skin cells divides constantly to replace surface cells that wear off. Once in a great while, UV damages the DNA in such a way that the cell can divide and divide without ever stopping, which is cancer.
Cells of the retina, also exposed to light, can be damaged in the same way.
I've used UV in the lab to kill viruses, and have studied the subject. I always used good eye and skin protection.
