Blood type resistance isn't so cut and dry, and AB blood type doesn't come up in the literature I found on the topic. It also kinda screams "I want only super rare people to get this!" which isn't the most solid worldbuilding base. AB blood type being mentioned is so common in fiction that it's a trope.
You also have to keep in mind that illness like this takes a certain degree of time to build up to "mass destruction", and in that time natural selection happens— so within a few generations, in the time the rest of the population is suffering, there'll be population growth among people who have AB blood type to a marked degree. But it's not quite so absolute as "everyone who doesn't have this blood type will die", seeing as the variety of blood types does survive even in areas that are heavily dominated by one type.
Pinging @StellaThomas again because I am not well educated on blood types and the resistance that happens with them.
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