I don't think that data supports the claim that Twitter is a one-way medium. What it shows is that there are a significant number of Twitter relationships which are one-way, but that doesn't mean that any individual is solely using Twitter as a receiving medium. What you would need is data on the number of users with no followers who are still reading their timeline; in the linked post, Dalton says this is high, but doesn't provide a link and I can't find data on this.