This summary is not the rule. The rule text controls.
The judge running a trial cannot also take the witness stand and testify as a witness in that same case. A judge's job is to oversee the trial and make rulings, not to give testimony about facts. Mixing these two roles would create obvious problems, since a judge can't fairly weigh evidence they themselves are presenting.
If a judge tries to testify in their own trial, the lawyers don't need to speak up or object to protect their rights on this issue. The problem is considered so serious that it's automatically preserved for appeal, even without anyone raising a complaint at the time.
