I would presume that "speaking in oracles even though understood by none" is intended to mean: "I will speak carefully and accurately and precisely in a way that conveys the truth to those who are capable of understanding," with the point being that I am going to speak the truth whether the people who are hearing me are capable of understanding it or not.
What's generally wrong with Oracles? Not that they are speaking with certainty and sounding profound, but that they are speaking with certainty and sounding profound about things that they cannot and do not know.
The things that Epicurus is stating are things that he maintains he can and does know.
Ultimately while we can do our best to speak clearly and in understandable terms, we have no way to compel the listeners to understand, or to give them the power to understand when they are under the spell of false ideas or otherwise incapable of understanding. All we can do is speak the truth frankly and let events follow as they will.
An important caveat as is stated by Lucian in Alexander the Oracle Monger -- we have no "duty" to speak to everyone, especially those where speaking will end up getting ourselves killed while accomplishing nothing.