When delivering a formal speech, you should never present a paradox to your audience as it may confuse them?

I agree with you, the statement is false. Decreasing the means by which you can give your audience a complete picture of what it is that you have to say is not a good idea. It's like thinking that your audience is too stupid or intelectually limited to understand the paradox or any other figurative forms of language. I would say that the case in which you should not use too many paradoxes or allusions is when your speech is directed to children or other audiences, for whom it may be hard to comprehend the meaning of your words. On the other hand, if the speaker is persuasive enough to say the paradox the right way or explain it nicely - there should be no problem. Obviously, the person who thought of this statement thinks every audience is stupider than he himself (or she herself.)