Most of the intellectual elite knew about Eritosthenes’s calculation of the circumference of the earth by Columbus’s time.
https://www.aps.org/publications/apsnews/200606/history.cfm#:~:text=Values%20between%20500%20and%20about,slightly%20less%20around%20the%20poles.
The gamble Columbus presented was that he thought it was much smaller than that.
He was proceeding from tales of Basque fishermen and others taking tons of codfish off some western lands. The Basque had reason to keep their discovery secret, like a protected secret recipe or doohickey for bizness.
There is an interesting history of the Viking Greenlanders. The author postulates that their disappearance was possibly due to the Portuguese (Basque?) recruiting of the limited number of Greenland males for fishing crews, and not so much climate change.
If there is any interest, I will do some more digging to find the title.