Sorry I may have been unclear, the quote means that when accused of libel a defendant can be exonerated if they prove the statement to be true, however damaging it might have been shown to be by the prosecution.

So to count as libellous a statement must be both false and have done provable damage to reputation. As such the easiest way to prove a statement is not libellous is to show it is true.

Rather than, how I think you interpreted it as meaning, that when libelled one should tell the truth in response.