In the 1300s, the strongest nation was in Africa. The explorers were in Africa.
This was not a secret.
Remember, at this time, Spain was under the control of the ‘Moors’, North Africans. Those ‘Moors’ in Spain, knew of Mansa Musa, knew of his pilgrimage to Mecca.
The report of Mansa Musa concerning the exploration of the Western Hemisphere in the 1300s was not written in strange hieroglyphics, but in Arabic.
Contact between the African rulers of Spain and the continent of Africa was continuous.
Columbus had to know, those who paid for his journey, had to know, that it had been attempted before, and that the fact a ship had returned, brought up many of the possibilities.