The likelihood is smaller than it used to be that disease will make it in via a false negative in the screening. However, probability doesn’t matter much if it is you who end up being the one in one thousand or one in millions who gets the misfortune. My best guess is that they expect very few people will get it so they can easily hush up the few that do with some payoffs if it does happen.
At 4% being mixed around that isn’t exactly “very few.” If it is going unguarded.