The people of Nineveh believed God; and they proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them.
It is a mystery how any student either of the Bible or of human nature could believe that Jonah's preaching, unsupported by any substantive proof, would be hailed in Scripture as a "sign."  Jonah's preaching would never have been believed at all, except for the fact that Jonah's deliverance from death was such an astounding wonder that "when the word came unto the king of Nineveh," he immediately believed every word of it! Without the prior miracle of Jonah's deliverance from death, only a fool could believe that the king of Nineveh would have come down from his throne, cast off his royal robes, clothed himself in sackcloth and ashes, and led the whole nation in repentance.  "Preaching" alone was never made the "sign" of anything!