When I was in college, my roommate and I got a cat. My roommate bought a book called How to Potty Train Your Cat and immediately set out to do this with our cat.
I was highly skeptical. I’d grown up with a lot of cats (our cat was actually one of a litter from one of my cats at home), and didn’t think it would work. Following the steps in the book, we first put the litter box next to the toilet. Over a period of many days, we gradually raised the litter box higher and higher off the ground by stacking books underneath it, until it was level with the toilet seat. The idea was to get the cat used to jumping up to do his business. We then used a special toilet “seat” for the cat, similar to the one that comes with this kit.. I don’t know where my roommate got the seat, but it wasn’t part of a set like that. Anyway, at first the seat had litter in it lots of fun to deal with when you only have one bathroom and then we removed it completely.
To my surprise, the cat continued to use the toilet and never had an accident or made a mess. Of course he didn’t flush, but at least we didn’t have a litter box to deal with! For the rest of that year, the cat had no problems using the toilet.
Unfortunately, after graduation my roommate took the cat home with her and after the change of scenery he refused to use the toilet. She tried repeating the steps, but it did not work and the cat would pee on the floor rather than use the toilet. It later turned out he had bladder problems and had to be medicated for the rest of his life, so maybe that had something to do with it as well. He’d use the litter box, but no more potty. :(
So yes, it is possible and I have personally seen it work. If we ever get a cat, I will certainly try to potty train it again!