Nope, your forgetting that a RP2 computer can change the signal "level" and first select what SET of 16, and then from any within that set.
For example, here's a vid that even shows that he knows how it works, as the clock in wrenched used the same concept
Also, as computercraft would be setting wires 1, 2, and 3 at the same time, it's trivial for the redpower 2 side to see that computercraft is requesting that you set book 7 in the frame. Because, lets face it, it would NEVER be sending books 1, 2, and 4 at the same time, the slot only can hold one book.