I remember reading once about an experiment that was said to make rats superstitious.

These rats were used in learning experiments. They would be put into a special cage and they'd have to do something to get a treat. Maybe they'd have to push a lever, or go to a certain spot. But they were pretty good at learning whatever they had to do. They were smart rats. They knew the score, they knew what the cage was for.

So they did a new experiment, where they put them into the training cage as usual. But instead of what they did bringing the treat, they always got a treat exactly 30 seconds after going into the cage. This continued for a while, and what happened was the rats each learned an individual behavior to bring the treat. One would go to a corner, another would turn in circles, another would stand up on its hind feet. And sure enough, the treat came. Their trick worked.

I imagine the society in Eliezer's story had something similar happen. Given the anthropic effect we are postulating, they don't actually have to do anything - a certain fraction of the worlds will get lucky and survive. But after it happens a few times, the survivors may well assume that what they were doing at the time their "luck" arrived was causative. In this case, they had a hero who seemed to get lucky. Maybe several heroes. And then somewhere they got the idea of summoning them from other worlds. After all, if they need a lucky hero to save them, they should get the luckiest heroes they can find. (I wonder what this hero had done to earn their selection?)

But there would be just as many other worlds, even instances of the exact same world, which have developed their own superstitions about what defeats the evil. They each carry out their rituals, and in each case, it works - for the survivors. We just choose to eavesdrop on a world which had a particularly interesting and amusing superstition.