I temporarily got caught in the Sad and Useless website, and found Science is Fun.
Most of the claims seem plausible, but I can't guess why cat urine would fluoresce.
Comments have directed me to Why Does Urine Glow Under a Black Light? which says:
You can use a black light to detect body fluids. It's actually a good way to look for pet urine or make sure a bathroom or hotel room is really clean. Cat urine, in particular, glows very brightly under ultraviolet light. Urine glows under a black light primarily because it contains the element phosphorus. Phosphorus glows yellowish green in the presence of oxygen, with or without black light, but the light imparts additional energy that make the chemiluminescence easier to see. Urine also contains broken down blood proteins that glow under a black light.
Certainly other mammals including humans have both phosphorus and broken down blood proteins in their urine, so is the proposed specificity to cat urine just an issue of concentration perhaps?