I'm only a first year so forgive me if I'm wrong, but a lack of evidence isn't proof of absence. It is possible that the Chinese simply didn't bring jade and porcelain products with them, or they took them when they left. You can't dismiss a theory out of hand simply without careful examination. I agree that with you though, that the roads and the walls do not look like they were made by masons, as you can similar rocks anywhere glacial.