I'm not sure I buy that, but either way you're not visualizing it quite right: both groups go on to have regular lives... it's just that one group drank an
additional 0.5 liters [1]. Like you said, death by alcohol is a cumulative risk, so that means each and every drink must raise your risk a teensy tiny amount.
How about this way: you agree that if you drink enough alcohol it will kill you, right? So you could surely find someone who almost drank themselves to death. Perhaps they got liver disease and came very very close to dying, but survived. So there must be a line somewhere. Given a huge enough pool of people, it should be possible to find someone who came so close to death that a single extra drink could have killed them.
([1] It's a thought experiment. The two groups lead otherwise identical lives besides the additional alcohol.)