Completely hypothetical scenario: Suppose arguments in a court case were dependent on the interpretation of the judge's decision in another case. Would it be legally valid to have the judge appear in court and testify as to the meaning of his original ruling?
In law, there is a distinction between questions of fact and questions of law.
Evidence helps sort out questions of fact. This happens at a trial court.
The trial court judge interprets and applies binding precedent from superior courts. If that judge makes an error in the interpretation of law, it can get sorted out on appeal.