As others have said, the concrete is porous, and the urine has likely well-diffused into and through the concrete if the cat was allowed to do this for a long time. Also, higher humidity tends to "activate" the diffused urine. You can google for more details on that. There are enzyme solutions for treating concrete, different ones for cat and dog. I would research those. And be aware, it can take weeks and months to treat a urine problem in a concrete floor, because the enzymes have to diffuse down to where the urine is.
Skim coats of concrete aren't going to help, because they will be porous, and the odor will diffuse right through them. However, as mentioned above, there are epoxy based floor sealants and coatings that might help. However, the proper prep will need to be done, as they may not adhere to spots where the urine is bad.
The previous owner of my house let her dog use the garage floor as a bathroom. We did enzyme treatment, had a floor wash done with some kind of nasty substance, and put down an epoxy based finish (2-parts I think). For the most part, that has solved the problem.