Well, y'all, here we are: our twelfth and final post on Dune. (I say "final," but you can count on there eventually being a series of posts about the various film adaptations. Plus, with a novel as good as this one, there's no such thing as having said it all; I didn't even scratch the surface, I just felt the itch and twitched my finger a bit.)
And it came to pass in the third year of the Desert War that Paul Muad'Dib lay alone in the Cave of Birds beneath the kiswa hangings of an inner cell. And he lay as one dead, caught up in the revelation of the Water of Life, his being translated beyond the boundaries of time by the poison that gives life. Thus was the prophecy made true that the Lisan al-Gaib might be both dead and alive.
Princess Irulan's epigraphs are almost always worthy of attention. Some are more worthy than others, and this is one of them. Consider the phrasing in that final sentence: "thus was the prophecy MADE true" (my emphasis). This implies that there was a scenario in which the prophecy was not true in and of itself, which surely misses a bit of the point of a prophecy.
Or does it? It's a matter that could be debated, and one could also debate whether Irulan is -- in her texts -- attempting to subconsciously hint that Muad'Dib really isn't all he's cracked up to be.