There would be two sources whereby the deception would leak out.
(1) Out of a whole nation there would be some who would bitterly resent the wiping out of every trace of true history and to substitute for it a tissue of blasphemous lies, which would degrade the whole nation. Men with pride of race and ancestry particularly strongly developed, and men too honest to stoop to such bare-faced deception would be found, who would put on record the truth of matters.
(2) The histories of the surrounding nations would have references to the children of Israel, in relation to wars, alliances, marriages, intercourse in trade and learning, and these could not be tracelessly wiped out. If it were possible for a nation to practice such a piece of colossal deception upon itself, it certainly could not persuade the surrounding nations to alter their histories in order to hide a deception in which they could have no interest.
And the further you get away from the time of these events, the more difficult and impossible would it be to persuade the people to embrace deception, and the more difficult to cover up every trace of their true history.