I don’t read Latin so I can’t really comment on that. The letter as a whole is about how people get wrapped up in learning everything which crowds out learning the most important lessons, which for the Stoics is virtue. Right before the quote I gave Seneca was discussing how the “liberal arts” don’t teach virtue, and one could argue that an education in the liberal arts was unnecessary for such.
In my reading of the English translation of the quote I gave, “letters” are being compared to the liberal arts. They are both something unnecessary for obtaining wisdom. When he says that “memory is more to be depended upon when it has no support outside itself” he is making an analogy with what some, at the time, may have believed that writing makes our memories weaker (since he was just discussing “letters”). This statement is an analogy of the effect of liberal arts education on wisdom/virtue, which is that one may become more wise by abstaining from the liberal arts to concentrate on philosophy.
Out of curiosity, how would you personally translate the quote to English?