When the barrier potential becomes more than the battery voltage, the current starts flowing in reverse direction. This current is very small.
In the "key point" they have explained that the minority current does not depend upon barrier potential. It means that if minority current is 10 mA at 30 degrees, then it will remain the same until the temperature is changed. So once the barrier potential becomes more than battery voltage, the minority current depends upon the temperature ie 30 degree, 40 degree etc.
"reverse saturation current" means that at 30 degree, reverse current is 10 mA. Even if you increase the barrier potential, it will be 10 mA and will not change. So reverse saturation current Io is 10 mA.