But this does not constitute a disagreement between them and humans about what is right, any more than humans, in scattering a heap of 3 pebbles, are disagreeing with the Pebblesorters about which numbers are prime!

That is an excellent compression of the arguments against the idea that humans and Pebblesorters are actually disagreeing about anything. (I don't think I found that unclear at any point, but this sentence has a distinctly and pleasantly intuition-pumpy feel.)