No one who witnessed this event could ever forget it. Faulty as human memory can be, there are events that burn into that memory such that we know what we were doing at a particular time. For example, those awaiting news of the first moon landing can typically describe where they were, and what they were doing. This event, of Christ’s appearance, was even more memorable.
They still did not fully comprehend what was happening, for the verse says that they “thought it was an angel that had appeared unto them.” Even though the Father had introduced the Son, Nephite understanding may not have comprehended that distinction as clearly as modern Christians do. They would learn, but in their current understanding they would more easily recognize Jehovah, their God, to be the heavenly voice, and that, therefore, it was another being who descended.