DeWitt on page 12 holds up the letter to Menoeceus as (alone of the extant letters) "composed according to the rules of rhythmical prose". Epicurus in this one letter is writing artfully. Perhaps that includes eschewing his customary synoptic introduction?
Regarding the same letter on page 46-47 he says this;
"Were it not for the survival of this piece we could not be so sure of his ability to write artfully, but possessing this we are justified in believing that other writings of similar merit existed."
So there's something about this letter in Greek that sets it apart stylistically, though if course it surpasses my power to say what that is exactly.