>How is a nun in middle of Africa meant to have even a basic understanding of how a specific government's budgeting and economic system works?
She can't have read a few books on economics? His explanation was pretty basic and nothing that couldn't be found in econ 101 textbooks. He essentially just assume she was uneducated.
When communicating with an unknown audience you can't make any assumptions of pre-existing knowledge. Especially if that knowledge is considered essential to the topic at hand. If you do otherwise it will leave the audience confused, or confident but with potential misunderstandings.
If this were in person, he might have been able to ask, "What do you know about the US's method of setting a budget and taxation?" and tailored his response after that. Since it was a letter, greater caution was needed.
Are you offended by the first chapter or two of nearly every programming language book that explains what variables, conditionals, arrays and the like are? If you know it, you can skip it. If you don't, you can read it to have a better foundation for the rest of the text.
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).
She can't have read a few books on economics? His explanation was pretty basic and nothing that couldn't be found in econ 101 textbooks. He essentially just assume she was uneducated.