Of course a cat is trainable. What makes a cat appear untrainable is the fact that it will perform what it was trained to do on the basis of whether or not it wants to do it. Because the cat is not a pack animal, there is no inherent need or desire for the cat to comply with anyone's wishes but its own. We humans have a difficult time accepting this because we relate as pack animals. A social group has a set of hierarchies and each individual has its place.
Dogs respond to peer pressure.
Cats do not. Because dogs can be bullied and intimidated into obedience, we expect that the cat should too. Thus making the cat appear aloof and untrainable. As an editorial note, I strongly disapprove of punishment in dog training. However, I mention these methods because they are standardly used by dog trainers who haven't learned that there are better, gentler and more humane methods.