Genetic drift occurs due to random chance, unlike natural selection. Presumably, the population that survived Justinian's Plague had beneficial mutations. While

Genetic drift occurs due to random chance, unlike natural selection. Presumably, the population that survived Justinian's Plague had beneficial mutations. While it is possible that 40 generations is long enough for genetic drift as populations mix and become heterogeneous, the surviving population was homogeneous in regards to those beneficial mutations. The bacterium Yersinia pestis never actually went away, so plague deaths continued uninterrupted during those 800 years, just not at the level of pandemic. Those mutations would continue to be beneficial and those with them continued to be actively selected for survival.