The whole dialogue is targeted specifically at decomposing the mysteriously opaque concepts of "truth" and "semantics" and "aboutness" for people who are having trouble with it. I'm not sure there's a part I could slice off for this question, given that someone is asking the question at all.

Maybe I'd ask, "In what sense are the pebbles not about the sheep? If the pebbles are about the sheep, in what sense is this at all mysterious?"

I make no claims about aboutness. Rather, I understand how the pebble-and-bucket system works. If you want to claim that there is a thing called "aboutness" which remains unresolved, it's up to you to define it.