Well, we also have to consider the Book of Prophecies. While Luxu himself did not have a peek at it due to obvious paradox concerns, I'm sure MoM would at least mention something world shattering like that. He was given a task and, judging by the book's existence, he succeeded. So that way he could be 100% sure nothing extra terrible would happen.
Even when the heroes were wiped out in the first iteration of the timeline.