Da'as Torah: Hashem does not allow any room for doubt! The episode of the Ketores should have sufficed ? He also made the test of the staffs, and was careful to remove any flimsy objection that they might raise [that because it was closer to the Shechinah, it produced flowers, buds and almonds, and all the other staffs, even those next to it, remained dry]. Likewise, all were healed at Matan Torah, except for Moshe's speech impediment, so that people should not say that we received the Torah from him because he was a gifted speaker.