Those letters were written in ancient Greek, the language from which the term "atheist" derives, admittedly. This is true. But that does not mean that the translation is as correct as you may assume. As religious scripture is never interpreted without a bias. And the nuanced meaning of words in their given language are lost when translated for people who do not actively speak in that language.
For example, I am fascinated by the ancient Greek term "logos", because in it's own language, it refers to a whole philosophical ethos that was unique to ancient Greek culture which I find amazing. And yet I have never managed to explain it clearly to anyone in my own language and culture even though I believe I do have a pretty clear grasp of what the ancient Greeks meant by that term.
Yet do I really understand it as they did? I mean, I think I do, but in the end I am not an ancient Greek. And I cannot go to ancient Greece and ask them if my understanding of that term, and of the philosophy that surrounds it is actually what they thought and felt about existence, God, and reality. So I can't really be certain that my grasp of the term is true, even if it is insightful, and appears to me to be relatively accurate.
The truth is that I could simply be substituting my own bias for what I believe they meant by that term. It's just impossible for me to know, for sure.
I have two particular enjoyments etymology and exegeses and I can assure you that what I have written is correct.