Despite it's entertainment value, there is nothing in the passage which supports the belief that the brain is a computer (or even a "meat computer") or demonstrates why the analogy of brain as computer is logically different from brain as mill. That's not to say that the complexity of a computer may not create a more attractive analogy, but one must keep in mind that no matter attractive Camelot appears on screen, it's only a model.
You quoted a statement Leibniz made about 300 years before Turing proved that there are machines that can compute everything that is computable. I and others explained in comments below, why we have reason to believe, that computers are fundamentally different from mills and that Leibniz's argument as well as Searle's fall flat. By the way, it is not the necessarily the "complexity of a computer that may create a more attractive analogy". The physical parts of computers can be less complex than those of mills. I wonder how Leibniz would have reacted if he had been shown Google. Would he have believed, for example, that the essence for its face recognition software must be "sought in simple substance"?
For more rebuttals of the Chinese Room argument see its Wikipedia page. I like this one in particular: A guy sits in a room and waves a magnet up and down therefore creating electromagnetic waves. But you don't see light coming out. So light cannot possibly be electromagnetic waves.
