It's not a sarcastic point at all. It's a logical argument. These denominations, all of them, claimed to be guided by the doctrine of Sola Scriptura. So do today's Protestants. Yet both groups arrived at contradictory doctrines. So how can this contradiction be reconciled if "the Bible alone" is to be the final arbiter of truth?
Because of faulty interpretation or willful misinterpretation.