The only person in the courtroom who must tell the truth is a witness on the stand. The attorneys (plaintiff, prosecutor, defendant) are not given sworn testimony. In fact there opening and closing statements are not testimony, they are meant to sway the trier of fact (judge or jury) to decide in a given way.
The judge when rendering decisions on objections, admissions of evidence; charging the jury and explaining the law is giving his/her interpretation, not issuing truths,