The sources available in the past were fewer than today, even if it was transmitted orally. The main test is the methods of learning from Scripture, and judgment. In that respect, it can be seen as similar to today. So is the Rabbi’s answer that what is said in the Talmud—that one who disputes his rabbi is as though he disputes the Divine Presence—applies only in a case where the student has not yet reached the level of issuing rulings? Because there is no mention of that. Of course, they learn it from those who disputed Moses, and one could answer that with regard to him they had not reached the level of issuing rulings. But the very fact that they derive it in a general way for any rabbi, without mentioning this, is difficult. And Korah and all the heads of the Sanhedrin there certainly would have reached the level of issuing rulings were it not for Moses.

However, I just saw that there is an interpretation that “one who disputes his rabbi” does not mean disagreeing with him in halakhic rulings, but rather setting up a competing study hall for himself, like Korah and his congregation, who created a dispute with Moses and separated from him. If so, then he is disconnecting himself from the tradition and from his connection to his sages, and that is the prohibition. Maybe that is a better answer according to the Rabbi’s approach.

Are there really no cases of people disputing their rabbis? Do you need proof for this? Didn’t the Ritva, the Ra’ah, and the Rashba disagree with Nachmanides? Even the Tur occasionally disagrees with his father, the Rosh.