Yes, cats have knees if we define the knee as a piece of anatomy which joins the thigh with the leg. The position of the cat knee is shown above in the picture. The philosophical question is whether cats have knees and elbows. I think the answer should be simple. As cats have legs and no arms then they have four knees rather than two knees and two elbows.
Some people ask whether cats have legs and arms. I think it is impossible to say that the cat has arms but perhaps a visitor will have a different opinion. If we say that a cat has arms and therefore has elbows we are, I would say, anthropomorphizing the domestic cat. We like to do that. It is understandable.