I said previously that they stumbled into the correct decision. By all accounts Harris is guilty of an elbow so no chance of an appeal, I'm no expert but I doubt "they said it was a stamp but it was actually an elbow so they couldn't have known" will hold much sway with an appeal committee.
It does seem to be based on the reaction which is awful refereeing. Perhaps I'm being harsh and the lino saw it but then why the delay in calling the ref over? I didn't see the incident but did see Smith lying prone for a while before the ref did anything, it's not the sort of thing a linesman would delay telling a ref.