And the example is not irrelevant since devastating pandemics have occurred throughout history.
I repeat my original point: Where did the original 1% - or 20% with the beneficial mutation come from? How did they get to that point? And for that matter, what distinction are you making between the 20% that survive and the 1% in your prior post? You need to untangle your own scenario first, and stop begging the question by conveniently assuming the prior arrival of the mutation population. It is that growth to near-fixation that needs to be explained, not assumed.
Moreover, note that you are using an absolutely extreme case (100% lethality to the non-mutants, with 100% exposure of the population to the disease in your example) to argue for natural selection. I agree that it is in those most extreme cases in which natural selection plays a role. It is in the 99.9999% of real-world mutations that are not so dramatic (or are lethal rather than beneficial) that natural selection fades into irrelevance. You don't need to be a creationist to understand this. Just study standard population genetics.