I interpret the scene a little differently. I don't think Genco actually believes that Vito can stop death. It's more of an old game between the two friends. Genco knew that Vito managed a lot of seeming impossibilities. "Pull some strings!" was one last compliment. But Vito was a serious man. He didn't want to hear foolish compliments in a solemn situation.
Another possible way to read the scene is that Genco was already half-dreaming, that he had lost touch for reality.

I think it would be absurd that Genco believed, Vito could stop death.