The prophecy is not “true”, nor are the Fremen the chosen people in the way I suspect you are thinking. The prophecy was put in place on Areakis in order to provide a possible opportunity for a Bene Gesserit agent in the future to exploit the people during a time of need. Similar (if not identical) prophecies were placed in worlds all across the universe. It was taken in by the Fremen and evolved along with their religion in unexpected ways, but functions as intended to allow Paul to assume control of the local population in a way that inspired fanatical devotion. But even as Paul and Jessica tricked the Fremen into allowing him to take control, he finds himself trapped in an ever shrinking number of paths as he is swept up by the Fremen‘a need/demand for Jihad, nearly all of which lead to calamity and doom for the universe. This struggle against a nearly inevitable future is one of the main themes of the series and in the end Paul proves himself to not be strong enough to make the hard choices to achieve the best possible outcome and gives in to the forces pulling at him (much of this does not become clear until subsequent books).