This is the "Argument from Incredulity by Implausible Consciousness Substrate"[0]. Other examples include Searle's Chinese Room, and the China Brain thought experiment (what is it with China?). To your credit, you've immediately spotted the problem - how is the actual human brain any different? It's also some sort of broadly mechanistic physical system, unless you're into divine spirits (but that just moves your problems).
The flaw is that there is no flaw, the only actual argument is your personal incredulity. And the reason your intuition is giving you bad results is that scale matters. The implausible substrates are far, far too small to realistically encompass the computation that a brain does, and this makes them poor "intuition pumps" (a useful phrase coined by Daniel Dennet). Your field of rocks looks more like a football field of sand, ankle deep. And of course, manipulating them one by one according to a tiny set of rules is absurdly distorting in time - to properly capture the sheer amount of parallel information processing, perhaps you should instead imagine them buzzing and vibrating and bouncing around and exchanging information and state with the grains around them in highly nonlinear ways. Does this start to sound a bit more of a plausible?
[0] A subtype of the "argument from bad analogy", which goes: 1) make absurd analogy, 2) point out that analogy is absurd, 3) wave hands