Rambam on TeShuva Before Yom Kippur

Rabbi Yaron Reuven brings a scary chiddush from the Rambam on the importance of TeShuva before Yom Kippur during the Ten Days of Repentance. Rambam says, the righteous are sealed in the Book of Life, the wicked are judged for death, but the benoni, the one who is judged in the middle on Rosh Hashana, must do complete TeShuva before Yom Kippur in order to be sealed in the Book of Life. The Sages ask, why doesn't he need just one mitzvah to tip the scale in his favor?