Indeed, I also think consciousness cannot be reduced to computation.

Here is one more thing to consider. All consciousness we can currently observe is embodied; all humans have a body and identity. We can interact with separate people corresponding to separate consciousnesses.

But if computation is producing consciousness, how is its identity determined? Is the identity of the consciousness based on the set of chips doing the computation? It is based on the algorithms used (i.e., running the same algorithm anywhere animates the same consciousness)?

In your example, if we say that consciousness somehow arises from the computation the man performs itself, then a question arises: what exactly is conscious in this situation? And what are the boundaries of that consciousness? Is the set of rocks as a whole? Is it the computation they are performing itself? Does the consciousness has a demarcation in space and time?

There are no satisfying answers to these questions if we assume mere computation can produce consciousness.