I am not even sure what the input/​output channels of a rock are supposed to be
I guess you imagine that the input is the physical forces affecting the ball and the output is the forces the ball exerts on the environment. Obviously, this is very much not useful for anything. But it suddenly becomes non-trivial if you consider something like the billiard-ball computer (seems like a theoretical construct, not sure if anybody actually built one...but it seems like a relevant example anyway).
I guess you imagine that the input is the physical forces affecting the ball and the output is the forces the ball exerts on the environment. Obviously, this is very much not useful for anything. But it suddenly becomes non-trivial if you consider something like the billiard-ball computer (seems like a theoretical construct, not sure if anybody actually built one...but it seems like a relevant example anyway).