Is that the part that makes you think this?
Well, yeah probably maybe… but that has nothing to do with your quote…
Quid est autem quare existimem non futurum sapientem eum qui litteras nescit, cum sapientia non sit in litteris? Res tradit, non verba, et nescio an certior memoria sit quae nullum extra se subsidium habet.
“Res tradit, non verba” is a Latin proverb that basically means “actions speak louder than words”. That’s more or less his main point and thus you might get away being “ignorant of letters”.
The “memory” that is “more to be depended upon” when it has “no support outside itself” refers back to “one who is ignorant of letters”. All he’s basically saying here, is that “it may be true” that a formal education in a certain subject may be more hinderance than help. That is what is meant by “no support outside itself”.
Ultimately, it’s actions (res) not words (verba) that matter. In a way, it doesn’t matter if you can debate Keynes vs. Hayek if you can’t apply it to improve the economy… (I’m taking liberties here to make my point.)
Nothing in that quote really refers to “the art of memory” though… not in Latin and really not in the English translation either.