I think it's a gray issue here.

Some caffiene will inevitably get into the blood, but the key here is that very little coffee is needed to stimulate the liver a whole lot as opposed to the amount of coffee that would be needed if it was taken orally. Only a little is needed because it gets delivered to the liver via this special circulatory loop that connects directly to the liver.

There are also other advantages of coffee enemas that have to do with the way caffiene is handled by the digestive system that I heard about in a talk by Julian Whittaker related to diabetes but unfortunately I don't remember the details. It was a while ago that I heard it.