Why believe it was a lucky guess when Joseph Smith said the Saints would be driven to the Rocky Mountains and become a mighty people?

On August 6, 1842, Joseph Smith recorded a prophecy about the Saints:

“Saturday, Aug. 6, 1842.—Passed over the river to Montrose, Iowa, in company with General Adams, Colonel Brewer, and others, and witnessed the installation of the officers of the Rising Sun Lodge Ancient York Masons, at Montrose by General James Adams, Deputy Grand-Master of Illinois. While the Deputy Grand-Master was engaged in giving the requisite instructions to the Master-elect, I had a conversation with a number of brethren in the shade of the building on the subject of our persecutions in Missouri and the constant annoyance which has followed us since we were driven from that state. I prophesied that the Saints would continue to suffer much affliction and would be driven to the Rocky Mountains, many would apostatize, others would be put to death by our persecutors or lose their lives in consequence of exposure to disease, and some of you will live to go and assist in making settlements and build cities and see the Saints become a mighty people in the midst of the Rocky Mountains.”
– Joseph Smith Papers, History, 1838-1856, Vol. D-1, p. 1362; Teachings of the Prophet Joseph Smith, 1938, 255.)

Joseph was right, but where would he have gotten this idea from?

See:

Nephi Lowell Morris – Prophecies of Joseph Smith and their Fulfillment

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