I'd just want to add that, I in general don't think it's a good idea to train cats to learn behaviors that are unnatural for them. One could say that, eating processed cat food and living with humans are already unnatural, which is true. But to the extent that's possible, I want to provide my cats the freedom to act as naturally as possible.

On this particular cat toilet training trick: cats are in fact quite particular about litterbox -- where it's positioned, how safe it feels inside it, how it smells. A human toilet is decidedly an unnatural setup for cats, and who knows what can happen. Maybe the cat learns to use it but constantly feels anxious about it (e.g. slipper surface). Maybe it will avoid pooping until it gets really uncomfortable. All of these can't be good for its health.