"Slim" it should be understood is classic Brit understatment. After all 99% chance of death does call for a certain amount of class when discussing it.
Note about us Brit's: The more desperate it gets, the more understated we get.
I believe it was one of Scott's men, on the ill-fated return from the South Pole, who, ailing and sick, arose and said, "I shall be gone just a little while." Whereupon he exited the tent,out into the storm, and was never seen again.
Talk about a stiff upper lip.....