The list shows both Leonardo's dilettantism in Latin and mathematics, and how printing changed the way we learn. Even in 1490, to learn "how to square a triangle", you did not have to get the master of arithmetic to show you. You could find a copy of Euclid's Elements and skim it up to Proposition II.14 [1]. The first printed edition of Elements (in Latin) was published in 1482 [2].
I mean Leonardo himself knew he was horrible in both Latin and math. He had access to various books in both subjects during his lifetime (thanks in part to the printing press) but his lack of foundational knowledge made it hard for him to learn geometry when he lacked skills in arithmetic and algebra.
Secondly, it's a bit funny that you say the book could have shown him how to square a circle, seeing as it's an impossible problem.
[1] https://mathcs.clarku.edu/~djoyce/elements/bookII/propII14.h... [2] https://archive.org/details/preclarissimusli00eucl