What is an understatement that expresses the affirmative by saying the negative of the contrary called?

elon musk, ceo of spacex, a startup space company is planning to head to mars by 2023. nasa is planning to go to mars at around 2025. both are just for a basic colony.

the narrator is a first person, participant one, who describes in the simple past tense important and difficult events in his earlier life. he also provides temporal markers as the month (january) and the time (12: 30 pm). finally the account of his experience is very detailed, to the extent that a participant narrator would talk the same way about his experiences. a good example of such detail is the mention of the “bovril ration” which historically was a native american food adopted by european explorers in arctic and antarctic regions due to its high level of calories.