"Death shall lose" is a false claim when the correct belief is "there is a certain chance that death shall lose and it is all the greater for my efforts!"

For the most part I agree with Thom in reading that as a declaration of intention rather than a knowledge claim, but I'll also point out that to a person who is familiar with the trajectory of science and not familiar with existential risks (which Harry might not be), "Death shall [eventually] lose" isn't a terribly unjustified thing to believe.