What he is afraid of is the age old dilemma for people beliving in reincarnation. Mahanama is worried that if he dies at a moment when his minds is muddled he will fall into a bad reincarnation.
If the answer is that a well trained mind rises and seperates out that would probably imply that reincarnation in a better place is meant. But if the mind after death becomes lofty and noble there is no rationality to that notion.