Some quick Google Books searches indicates that Sister Mary Jucunda might have been a nun for maybe as much as 20-30 years by then (someone by her name shows up in various documents from the Catholic church dating back quite far). While I have no idea how much of her life she spent in Zambia, it is quite possible that she received her "standard education" in the 30's or 40's.

It would seem quite reasonable for a nun who received her regular education that early to possibly have grown up going to a religious school somewhere where the idea of teaching topics such as economics to a woman would not have been particularly favoured, and/or where the use of a "standard library" to read texts outside of the curriculum would have been discouraged.

In other words, depending on the wording of the original letter, it might very well have been entirely reasonable to assume that she would not have had any knowledge of economics (for that matter, you could assume a fairly substantial percentage of adults today would benefit from the explanation he gave).